These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Draco in Leather Pants: We have Galacta Knight, who was so powerful and dangerous that he had to be sealed away. Sounds like some horrible villainous character, right? Exactly! That is why every other fangirl draws him as an overly-cutesy, baby-faced sweetie that loves to tickle Meta Knight.
And, of course, there's Marx. Okay, so he did incite conflict between the sun and moon just to trick Kirby into summoning NOVA, granting him the power to rule Pop Star and do whatever he pleases, but...but he's so cute! There have been a number of fanfics either attempting to redeem him or altogether ignoring that he was evil in the first place.
Ensemble Darkhorse: Galacta Knight immediately gained a huge fanbase after Ultra was released.
Marx is generally agreed to be the most popular villain in the series after Zero/Zero Two.
Among regular bosses, Heavy Lobster proved popular enough to get his own theme in the remake, appear in the anime, and appear as a boss in the Kirby Brawlball subgame in Kirby Mass Attack.
Game Breaker: Stone Kirby in Ultra completely annihilates the otherwise Nintendo Hard True Arena if you're willing to fight cheaply and spend a lot of time in rock form to keep from being hurt.
The Plasma and Hammer Ability can count as this as well. Plasma by virtue of being the only power with a projectile that goes across the whole screen (provided you've done at least a little charging), and Hammer because it's just so damn powerful.
Hammer's greatest strength is unlocked for Kirby in Milky Way Wishes; when using the dash+A technique, Kirby sacrifices his Hammer ability For Massive Damage, but because of how abilities work in MWW, you can just re-equip Hammer and do it again almost immediately.
In Milky Way Wishes, the Copy ability, which is ordinarily pointless for Kirby to have, becomes this by allowing Kirby to bypass the gimmick of Milky Way Wishes and get his ability to copy back. Consequently, the Copy ability is very well hidden.
TAC may also be one of the better helpers. His attack is lightning fast, does surprisingly high damage, and his guard makes him completely unhittable. Also, he fully heals when copying an ability.
Even scarier when you consider this: that scream emulates his rage at losing again to Kirby after who knows HOW long of burning in the pit, his fear of death, his pain as he dies. Definitely Nightmare Fuel.
The cutscene preceding the Marx Soul battle can be pretty chilling as well, seeing as it shows Marx's corpse drifting through space, eyes staring blankly and skin drained of color. Then he's engulfed in darkness and transformed into a monstrous revenant. Brr.
Some found Wham Bam Rock (especially his original design) sort of creepy looking. His new design looks like a floating mask, his old design looked like a bunch of organic facial features floating in the dark.
Tear Jerker: The ending of Revenge of the King. Poor Dedede. At least, it starts off that way, but becomes a heartwarming moment when his Waddle Dees come up to console him.
That One Boss: Meta Knight. He's incredibly fast, has a lot of hard-to-dodge attacks, has a good A.I. and is a small target. In The Arena, you can count on him to be the one to kill you (if you're just starting out) or force you to use a Maxim Tomato (if you're already experienced).
Fatty Whale counts as well due to his massive size taking up most of the screen, and his tendency to bounce around.
Butterfingers/Jukid, for being the only boss that has Kirby lose his current ability with one hit, when other enemies and bosses don't require the specific number of times that Kirby is hit in order to lose his ability.