5. Empress Bulbax - Pikmin 2. Specifically, the second battle when she goes from a pushover who rolls from side to side to a destroyer of worlds who spawns dozens of babies in seconds. The babies are easily killed, but they're fast and are the only enemies in the game whose bites instantly kill Pikmin. No matter how many times I tried, no matter how many Pikmin I managed to keep alive to the end of her cave, those babies always cut my armies down. I don't believe I ever actually beat her.

4. Magnus von Grapple - Paper Mario: The Thousand-Year Door. Lots and lots of health, high defense, punishing attacks that are really hard to block, the works. What always made him extra frustrating for me, though, was that you have to fight him in the chapter before you start getting any of your high-damage partners or abilities; and the partner you get in that chapter and are encouraged to use, Flurrie, is arguably the most useless combat partner in the game. I had to hammer away at it and try different tactics for weeks before I could beat it the first time.

3. Shao Kahn - Mortal Kombat 9. Even for a fighting game final boss, he's absurd. Only a few of your available attacks have any chance of damaging him, and those only take a few pixels off his health bar -- all the rest glance off against his arbitrary super-shield. His attacks in turn can take off thirds or even half of your health, and many can't be properly blocked. And to top it off, you have to play as Raiden, a character I've always found sluggish and imbalanced and thus never bothered to master. Took me weeks.

2. Dark Bowser & Dark Fawful Core - Mario & Luigi: Bowser's Inside Story. It's actually a really fun, well-designed, and rewarding final boss fight, but the absurdly high health of both enemies and their ability to restore almost all health at random progressively wore me down until I started giving up for long periods of time before coming back and flailing at it again. It took me almost a year and a half to actually beat it, and the successful battle took around two uninterrupted hours.

1. Vox Airships - Bioshock Infinite. I don't care that it's more of a long wave-based escort mission than a boss fight, because it's the closest thing the game has to a final boss and the single hardest fight in any game I've ever played. Wave after wave of the most punishing enemies and minibosses the game has to offer storm the arena to home in on a generator with pitifully little health, and you have to stop them from destroying it. Unfortunately, your options for defending points you're not at are limited to single-use and unreliable vigor traps, and the endgame ability to summon Songbird that's supposed to make you feel powerful takes forever to recharge, so you pretty much have to save it for the scripted "destroy the airships" and spend the rest of the time desperately bouncing around and trying to be in ten places at once. The fact that it's a gate blocking one of my favorite endings in gaming still feels like a personal offense.