Stan Lee thinks big. He came up with Galactus, a massive purple guy who eats entire planets. That's menacing! To get any more epic in scope, you'd have to have Andromedax, The Galaxy Who Shoots Other Galaxies With A Big, Galaxy-Sized Bazooka. Even better, Galactus isn't some sort of hand-chafing nefarious schemer. He's just very large, very hungry, and loves the great taste of ecospheres.
The monstrous plant Kaiba from the anime series Kaiba consumes planets in addition to the memories of their inhabitants. At least two characters think this is a good thing, viewing it as a form of Instrumentality.
In Psyren, the membrane that surrounds the planet is actually a being which reincarnates by eating planets.
The Magical Girl Lyrical Nanoha A's The Movie Second's version of the Darkness of the Book of Darkness, NachtWal, which doesn't just destroy the planet its on but consumes it, piece by piece, growing larger the more it consumes, eventually swallowing the planet whole.
The Original Life Fiber from Kill la Kill. Much like Lavos below, it technically doesn't eat the planet itself, merely draining energy from the living things instead; also like Lavos below, once it's consumed its fill of the planet's energy, it spreads its children to other planets, destroying its host world in the process.
One of the two Trope Makers is Galactus, from the Marvel Universe. His appearance has varied significantly. In main universe, he's a non-abstractuniversal function that resembles a giant of the same species as any viewer but a hundred times bigger, in Ultimate Marvel he's a swarm of robots that both needs to eat planets and hates organic life, and in the second Fantastic Four movie he's a formless mass although if you look really closely, you can make out Galactus's helmet within the swirling clouds at its core.
"Of all the creatures in the vastness of the Universe, there is none like me. I was present at the birth of the Universe, and I shall be there at its end. Though I ravage worlds to live, I bear no malice toward any living thing. I simply do what I must to survive. And why must Galactus survive? For, no matter how many worlds I devour... How many civilizations I destroy... It is my destiny to one day give back to the Universe – Infinitely more than I have ever taken from it. So speaks Galactus!"
Then Marvel introduced Galacta, Galactus' estranged daughter. She has the same cravings her dad has, and is capable of devouring planets, but she keeps this urge in check by working as a superhero, feeding on malevolent forces of energy (which would be destructive if left unchecked). In between those feedings, she spends a lot of time trying to control her insatiable hunger by eating bacteria, mostly because anything more filling and she might go on a feeding frenzy that would destroy the world. However, she's pregnant, and her baby has the same cravings even now, while still in her womb.
Every time the Celestials succeed in this, it destroys the universe. Which doesn't bother the Celestials, since they can survive it, and this is the only way that they can make more of themselves. Galactus was revealed to have originally been a survivor of a previous universe that the Celestials destroyed, and dedicated his life to stopping them from ever succeeding again.
The main universe Galactus, despite being pretty omnicidal, is vital to the survival of the universe. He's a living seal keeping an Eldritch Abomination locked up, and without him, it'd destroy the universe very, very quickly. Most of the energy from his meals go towards the seal rather than his own body, which is why he's always so hungry.
There's also Hunger, who was sort of like Galactus, except it fed on universes.
DC has a largely-forgotten parody called Mr. Nebula, who doesn't eat planets, just re-decorates them by wiping out all life. He is placated by being given Las Vegas, which is about what he's going for.
Marvel also has Ego the Living Planet, who is himself eaten by cosmically powered zombified superheroes in Marvel Zombies.
In the Marvel cosmic miniseries "Annihilation", the extradimensional invaders fielded a gigantic, organic, living spaceborne weapon called the "Harvester of Sorrows" which destroyed entire worlds and consumed the remains. What was left of the organic life was separated out so the crew could enjoy it.
Then there were the Infinites - beings from beyond the universe who wanted to rearrange the galaxies to 'improve the energy flow of the Multiverse'. To this end they demolished planets and re-shaped them into the means to move galaxies. The actual demolishing was done by human-sized drones which destroyed everything in sight and used the raw materials to replicate themselves until there was nothing left except the molten core and trillions of drones. The Infinites were pretty appalled when they discovered that planets - which to them are about the size of grains of sand - have tiny beings living on them which can't survive that sort of thing.
Interestingly, the first appearance of something called a Sun-eater was very different from later interpretations, being a fiery giant that new legionnaire of the month "Marvel Lad" really Mon-el having fun took out in one page as part of his initiation test. The sun-eater gained it's much more formidable cloud form later.
Additionally, Legion Lost introduced the Omniphagos, which, as the name implies, ate everything, and had to be kept imprisoned or it would eat the entire universe.
The World Devastators from the Star WarsExpanded Universe (specifically, the Dark Empire comics) are a variation on the Planet Eater concept: nigh-invulnerable machines that tear planets apart with powerful tractor beams, thus mining raw materials to build huge automated fleets of starfighters and, potentially, more World Devastators. While it would take a very long time for a fleet of World Devastators to completely consume a planet, they don't have to eat the whole thing to depopulate it.
Dark Phoenix in X-Men proved even more dangerous than Galactus considering she got sustenance from stars, causing them to go nova, which destroyed one populated planet. The Shi'ar and their allies realize that while Galactus is bad enough, having a menace that destroys whole solar systems is something they have to move against at any cost. And eating stars is only the beginning; Dark Phoenix had the potential to destroy the entire universe, and in some alternate continuities it did.
To the point that to this day, some Shi'ar aren't over it. Jean's entire family was slaughtered by the Shi'ar Death Commandos, (during a time Jean was mostly dead, so it's Rachel, not Jean, who swears vengeance for the death of her family. Apparently it didn't occur to them that pissing off a Phoenix host is a bad idea.) and in Phoenix: Endsong, there was a Shi'ar operative who came to Earth when Phoenix woke up. Even though they know that it's highly unlikely that something like the sequence of events that led to Dark Pheonix could happen again (Jean is a pretty decent person who almost never has full Phoenix powers, and Dark Phoenix was the result of Mastermind screwing with Phoenix's head for months. Throw in the Continuity Snarl retcon soup about how much of Jean was in the original Phoenix Saga and how much of the Phoenix is in Jean after it. It's not the textbook With Great Power Comes Great Insanity case and it certainly wasn't Jean's fault.) In universe and out of it, she's the poster child for Never Live It Down.
In The Return of Hanuman, a monster formed from a volcano named "Armageddon" is supposed to eat the contents of Earth entirely if Hanuman didn't stop him.
It's not alive, but the gigantic Green Sun from The House on the Borderlandconsumes every planet in the solar system, and the sun as well.
The Roar from The Silver Sequence is a member of a species that eats planets, or at least all the life on them.
The Forerunners in The History of the Galaxy series feed on all forms of matter from space dust to planets. They are not particularly large but are able to breed (through mitosis) at tremendous rate after eating. The Forerunner Crisis occurred 3 million ears ago, when a swarm of these low-intelligence creatures was moving through systems attracted by starlight and leaving only stars with no planets in their wake. They were finally stopped by a species-wide Heroic Sacrifice of a Precursor race. One of the novels reveals that the Forerunners are, in fact, creations of an Energy Being, designed as its vessels. They were created with prototype DNA molecules. Some of them died on various planets, resulting in Panspermia. They weren't designed to go crazy, though.
In IT by Stephen King, the titular monster claims that it is "the eater of worlds." We're only shown it eating children, but considering what almost happened in the end . . . It is female and she lays eggs. Hundreds of them. We're lucky for Ben and his cowboy boots.
The main character in the short story Thang eats planets.
In Keith Laumer's Retief story, "The Garbage Invasion", the Basurans want to take over the paradise planet of Delicia so they can eat it, the way they did their home planet.
The organisms in the book Moonseed by Stephen Baxter. "It eats planets. And it's here".
Doctor: The Racnoss are carnivores - omnivores. They devour whole planets.
The Great Vampires from the Tom Baker era arc "State of Decay", supposedly devoured planets as well.
The titular Cool Ship from Lexx loves (and needs) to do this. The Lexx even took a few bites out of Earth.
In season two, Mantrid went one better by eating the entire universe. Technically, dismantling it and converting it into his drones, but they are arguably a part of his body.
In a twist, the Lexx actually receives less nourishment from doing exactly this, as it needs protein from organic material for sustenance; this is why the Lexx only eats Holland instead of the moon, it's just that 'accidents' happen and the rocky ("not very tasty") remains of an exploded planet are all it has to make do with.
Stargate SG-1 has the Replicators who, after getting access to a time-dialation device, were able to entirely cover a planet with replicator blocks. Granted, the planet is still there, but it's less of a planet now than a planet-sized mass of replicator blocks.
To be more specific, they don't eat the planets themselves, but they may as well have. Instead, they consume every last scrap of matter that can be converted into biomass to make more Tyranids, as well as draining any oceans and sucking up the atmosphere. Planets worked over by Tyranids still exist, but are nothing more than utterly barren chunks of rock.
Draedens, multi-tentacled monsters from the Immortals boxed set, aren't able to literally devour planets whole, but they're entirely capable of munching a world down to bedrock.
Gammaroids, great turtle monsters from Spelljammer, tend to cause tectonic activity during hatching and then go away, but a big infestation can gradually (over centuries) destroy planets, as happened in Moragspace sphere featured in SJS1: Goblins' Return module.
In the Yu-Gi-Oh! game, there's a high-Level Synchro Monster called Star Eater; if its name is any indication, it takes this Trope up one notch further. (Given how powerful it is, it's possible; it could override the effect of Number 9: Dyson Sphere.)
Final Fantasy VII's Jenova, who combined the mimetic/assimilation abilities of the creature from The Thing (1982) with the retirement plans of Lavos. Much of Jenova's history is kept vague: it was discovered in a glacier near the ruins of the ancient Cetra civilization. It was frozen in the shape of a naked woman, except with missing limbs and numerous alien appendages jutting out from its shoulders and torso. According to lore a "calamity from the sky" came to the planet, imitated human form and nearly wiped us out until a few remaining Cetra buried it in ice. It would have remained dormant but for an excavation team in the present day, who were looking for remnants of the Cetra and assumed that Jenova was a mummy. Eventually, Shinra's scientists took possession of the body and began experimenting on injecting Jenova's undying cells into potential soldiers, which mistakingly allowed Jenova to work it's will from inside them. Its ultimate goal is to destroy the planet, then use the hollow shell as a vehicle for preying on other worlds.
One of the entries in the Gradius series, Life Force/Salamander, has one of these as the storyline: you have allowed yourself to be swallowed by Zelos, a creature so large it eats entire solar systems, so that you can fly through his body and take down his organs one by one with your heavily armed spaceship.
Parasitis, from Abadox, digested the titular planet and assumed its shape. Parasitis obviously takes the maxim "you are what you eat" too seriously.
Pyron from the Capcom game Darkstalkers is similar to this in most regards, although he usually prefers wiping all life off the face of the planet and letting a new race come into power before starting it over again.
In Chrono Trigger and Chrono Cross, Lavos is gradually eating the planet's energy from the inside, having first attached itself in 65,000,000 B.C. What makes it particularly nasty is that Lavos is heavily implied to alter evolution on the planets it lands on, so that local sentients become more intelligent and advanced. Why? That makes the meal more filling.
Cross expounds on this to make matters more complex: Lavos enhances the DNA of the planet's native life, creating sentience, then incorporates that DNA into its offspring, killing the planet so that its evolved young can move on to other worlds, continuing the life-cycle. This is a very literal take on the Panspermia theory of life propagation, with Lavos as the sperm that penetrates and fertilizes the egg, Earth. This kind of sucks for the Earthlings in the long run, but after all you can't hatch an egg without breaking it.
In Final Fantasy IX, Terra is a parasitic other planet gradually dining on the souls of Gaia, a planet it has been slowly trying to devour for a very long time.
In Wild ARMs 2, the Encroaching Parallel Universe, Kuiper Belt, is gradually devouring not only the planet Filgaia, but Filgaia's entire universe, in what is termed the "Stain Paradigm".
In Xenogears, the removal of the Limiters from the planet's population causes most of them to suddenly transform en masse into Wels, and it is revealed that all humanity on the planet was allowed to grow and thrive so that they could be consumed as raw materials for Deus.
In Xenosaga Episode II, the activation of Proto Ω on Miltia causes it to instantly devour the planet from the inside out for raw materials, and then literally hatches out of the planet like an egg, throwing off pieces of its crust like egg shells.
The Old Gods from Warcraft are planetary parasites that merge themselves to a planet and slowly corrupt it. Whether this would eventually destroy the planet is unknown, as the Old Gods on Azeroth were sealed away by the Titans (they could not be killed, having already corrupted the planet to such a degree that removing them would've required the destruction of Azeroth).
The Star Forge in Knights of the Old Republic is a variant of this - it feeds off a star to create vast swathes of technology for the Sith army. Whether it would eventually destroy the star it's using for energy and raw material is unknown (the star it's found orbiting seemed undamaged, and the math at least suggests that it's unlikely)
Darth Nihilus in Knights of the Old Republic II is (or was) a person who consumes the life-force of planets through the Force, leaving them barren rocks in space. It turns out this is pretty much all he does, since his hunger is all-consuming and all that motivates him.
Some Space Monsters from the first and second Master of Orion games will eat planets if you don't stop them, leaving behind only "a barren, radioactive husk".
USG Ishimura in Dead Space is a humongous scraper capable of gutting a whole planet and collecting its mineral resources. So basically it's a human-made planet eater/coal miner. Repeat that to yourself and realize the awesome.
Dead Space 3 takes this Up to Eleven with the Brethren Moons, Necromorphs the size of moons that reproduce by consuming all biomass on a given planet's surface.
Guhnash from Fossil Fighters, a gigantic space-fish-Pac-Man creature, presumably of the physical-devouring variety of planet eaters. Around 600 million years ago, it ate the dinaurians home planet. Thanks to their subsequent attempts to 'correct' the evolutionary path of Earth, it starts making its way there.
The Toronto is another example of man made Planet Eater. It's basically a giant metal colossus that settles on the planet and digs up the surface, while slowly expanding. Eventually it could reduce the entire planet into raw materials.
The planet Fusion is this in Cartoon Network's Fusionfall.
Phaaze from Metroid Prime is, itself, a living planet however, it sends out its offspring to corrupt and devour other planets as a method of reproducing.
The entire game Life Force takes place inside one of these. Half the levels are organic themed, while the other half are apparently the surface of the actual planets that were consumed.
Parodied in Casey and Andy with the Planet Devourer, which can eat planets... eventually. (It's about the size of a canary, with a similar rate of digestion.) It does eat Mars at one point... except it's Only a Model.
The power granting Entities of Worm don't literally eat planets, but they do regularly obliterate them to make use of their mass energy as part of their life cycle.
In Terren the goddesses Pura Velpormia and Hellmasin Miastrius swallow heavens and hells, respectively, whole. Unless they decide to wear them as jewelry instead. The creator also draws some macros that grow big enough to eat planets in a more conventional sense, but one of them was still nothing more than a snack◊ for Velpormia.
There is a SCP capable of this, but most scientists aren't even aware of it. It's SCP-2317, and only O5 personnel are fully aware of it. Why? There is no way to stop 2317 once it awakens.
Dial M for Monkey brought us the Galactus parody Barbequor, who tried to turn the entire solar system into a shishkabob.
Coop battles one of these in Megas XLR, after it has the gall to disrupt his wrestling show. He destroys it by feeding it a missile packed with soda and Pop Rocks. And that's by accident. His first attempt — a Macross Missile Massacre — is incredibly effective at devastating an area the size of New York City, on a creature the size of Earth. So then he tries smashing it...
The Beast Planet from Shadow Raiders gets along by eating planets. Oddly, it's also an Implacable Man... err, planet, as it survives several is completely unfazed by ALLattempts at destruction, even eating two of the heroes' planets. The Beast also produces a near-endless supply of drones who tend to kill off anyone escaping from the Beast's feeding.
In the 90s Spider-Man animated series, Dr. Strange villain Dormammu could devour entire dimensions.
Hungorto, in Duck Dodgers is Galactus, right down to having a cosmic-powered Herald. He is defeated when the combined forces of Earth and Mars throw all their food at him.
Seemingly played straight but ultimately averted with The Darkness in The Fairly OddParents three part special, it eats planets, but it's just defending itself because the denizens of the planets he's passing by freak out and attack it for looking menacing, when it's all sorted out, it spits out everything/everyone it and its termi-errr, eliminators ate.
This theme was revisited in Star Trek: The Animated Series with a gaseous antimatter entity the size of a small nebula which arrived from outside the galaxy and began digesting planets. In a twist on the theme, the entity proved to be sentient, though not particularly bright, and once the ship's crew managed to establish communication and explain that its next intended meal was inhabited by living things, the entity decided that eating inhabited worlds was murder and set out for uninhabited regions.
Black holes, if they get close enough.
Near the end of some stars' life cycles, they will expand and envelop nearby bodies, including planets. This is the likely end of Mercury and Venus, considering the type of star we orbit. Earth is an open question; some astronomers suggest that the sun won't quite reach earth's orbit of the time (somewhat farther out than now).