The battle against Marx in the final part of Kirby Super Star. It's even more fun if you try to defeat him without using any abilities whatsoever (it is possible), which ups the awesome factor even more.
Marx SOUL from the True Arena. It's Marx, more powerful, more awesome, and most of all, more insane. His maniacal laugh, crazy rolling eyes, and final scream as he falls apart all make him both a Most Awesome Boss and (relatively mild, but still) terrifying. Until you hear his deathscream.
The boss battle against Miracle Matter in Kirby 64 is a battle against a bizarre shapeshifting 20-sided die who is capable of mimicking all of Kirby's abilities in some shape or form who can only be damaged by whatever ability he's mimicking at the time. (Good luck beating it if you haven't figured that out.) There's seven different forms he uses to attack you, some of which are honestly pretty hard to avoid, he switches forms at random, and he can only be hurt (and even then it's only a slight chip off the HP bar visible at the bottom) when he's in the middle of attacking you. Still one of the lengthiest, most interesting fights in the whole franchise. The music's pretty rad too. Unfortunately, it's not on the OST.
Computer Virus is probably one of the most hilarious bosses of all. Sure, it ain't the hardest of all the bosses, but anyone who's got any experience playing RPGs will have a chuckle at it. Puffiness, Level Up!
Anytime you fight Meta Knight, especially when using a sword.
Galacta Knight, the final boss of Meta Knightmare Ultra. Think Meta Knight as an angel... a really powerful angel. Oh yeah, we did mention that you fight him as both Meta Knight and Kirby, right?
He returns in Kirby's Return to Dream Land as a Bonus Boss, being fought in the True Arena as the third to last opponentnote though the two opponents after him are the same guy, so he pretty much fulfills the same role as in Super Star Ultra's True Arena. Unless you saw his picture in the end of the Extra Mode Credits, this fight is only foreshadowed by easily missed details in the appearances of Stone Kirby and Metal General. The fight itself is... mediocre, at best, until, like many bosses in the game before him, he turns red. His theme music kicks in and he starts wiping the floor with you with upgraded versions of his previous attacks and throwing out new attacks. Good luck going through all the previous bosses after you die, one of which is the OTHER Bonus Boss.
Speaking of the Metal General, he's pretty good. He fights like Meta Knight, but uses his own, mechanical attacks, as well, including generating giant rockets that he uses to ram into you. And the other Bonus Boss? In Extra Mode, after beating the Metal General EX, he summons HR-D3, the robot from the Kirby Quest minigame in Mass Attack, which he redesigned in his image. That's right; he managed to take Dedede's own Humongous Mecha. And, considering the first form, it's highly likely it was designed after Kirby 64's HR-E.
Squashini was also a pretty good fight. Especially with the giant roulette (when it isn't completely aggravating).
0^2 from The Crystal Shards was some seriousMood Dissonance with almost the entire rest of the game, considering the Awesome Music that served as the BGM, the Amazing Technicolor Battlefield (dark and drab unlike the backgrounds of pretty much every other level in the game), and the Unexpected Genre Change to a rail shooter. And the boss itself is a giant, winged eye with a halo and a sickeningly-green tail. The eye is also bleeding.
Its original incarnation, 0 from Kirby's Dream Land 3, attacked by shooting its own blood at you. And this is a Kirby game we're talking about.
Drawcia from Canvas Curse/Magic Paintbrush. Tough as nails when you first fight her, especially if you have no clue what you're doing. Once you've figured out how her attacks work, though, she's an absolute blast to fight against, and easily worth playing the game again and again just to repeatedly fight her.
The Cloaked Nightmare at the end of the second game, especially considering the race against time for his first form, his demonic second form, and several strong attacks (then again, he doesn't look as imposing in the GBA remake).
Any of the bosses from Revenge of the King count, mostly due to them taking a level in badass, but special mention goes to Kabula. After going through Crush Clouds, you'd expect to go straight to Castle Dedede again. Nope. As you're flying to Mt. Dedede on the warp star, he's shot down by Kabula. In a last-second save, the three stars you've collected from the bosses form the Starship from Milky Way Wishes to help you destroy Dedede's warship. In fact, this is one of the few bosses that gets special pause screen text, which means the game itself acknowledges the awesome!
While not up to the level of other Kirby bosses, the final battle of The Amazing Mirror culminates in a shoot-em-up, and once you bring its health bar to zero, the credits roll and you can still beat on it.
The battle with Dark Meta Knight before fighting the final boss is incredibly cool too. Then again, anything involving Meta Knight is cool.
Amazing Mirror also introduced Mega Titan — one of the few bosses in the franchise who cannot be damaged by the conventional "Pick any ability and attack relentlessly" method. Unless you enter the fight with the (rather underpowered) Spark or Beam abilities, the only way to beat Mega Titan is by smacking him into the electrified walls of his arena. These same walls also happen to instantly destroy any loose ability stars, should you have your ability knocked out of you by one of Mega Titan's difficult-to-dodge attacks.
Dark Mind's star barrages nearly put Nightmare to shame, and his bouncing lasers off of his mirrors is an extremely chaotic and tough-to-dodge attack. For such a confusing game, beating the snot out of this Mirror Zero is a very satisfying way to cap it off.
It really only counts because of the nostalgic value, but the final boss in the original Kirby's Dream Land against King Dedede is incredibly awesome. It's what started the rivalry, and all.
Fighting Dedede is generally awesome all around, but perhaps the crowning one was in the Adventure remake, Nightmare in Dream Land, mostly because of how HUGE he is in this one.
Kirby's finishing move against Magolor. So much Sword. And after you barrage him with that Ultra Sword, he goes even more One-Winged Angel and turns into a big, black sphere of darkness with big red eyes. His stomach is pure white with an eye that resembles Dark Matter. This is without going into the Extra Mode version. See for yourself...
Goriath, also from Kirby's Return to Dream Land, is an excellent boss battle, paying homage to several famous manga such as One Piece, Naruto, and Dragon Ball. His EX fight even has a Spirit Bomb attack!
From the same game as the last two examples, there's Grand Doomer. Basically a King Mook for all the Sphere Doomers. He comes out of nowhere and uses attacks unlike any other enemy (other than Doomers), and the fight ends with Grand Doomer shielding itself so that Kirby has to use the Ultra Sword on him. On top of that, he's the game's final boss before moving to Halcandra.
Kirby's Dream Collection gives us Smash Combat Chamber EX, a Boss Rush with every miniboss from Return to Dream Land and Whispy Woods EX. Kirby fights them with the Smash Ability, which is basically his SSB moveset, making a return from Kirby and the Amazing Mirror. And the songs that can play include the True Arena boss theme from Kirby Super Star Ultra and Miracle Matter's boss theme from Kirby 64: The Crystal Shards. Alone, they're great. Together, it's amazing.
And immediately prior to that, Masked Dedede's Revenge. Think Masked King Dedede with a massive power boost and a battle axe 3 times his size. And just wait until you see his Mirror World counterpart, Shadow Dedede.
This game's answer to Galacta Knight is Dark Meta Knight from Amazing Mirror, and he's just as awesome. Theme Music Power-Up and everything.
Kracko's back, and he's not messing around this time, with awesome new attacks.
The second boss, Paintra, falls into this, especially for everyone who's played Kirby: Canvas Curse.
The fourth boss, Coily Rattler, has an entire thread on GameFAQs devoted to gushing over it.