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) Nightmare Fuel.
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?
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 mentioned above? 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.
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.
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).
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.
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 Dreamland, 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.
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 Dreamland 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.