Brineybeard was the second hardest for me. The absolute hardest? The mad scientist with the giant robot. I hate looking up help online, but for him, I not only looked up help, I looked up a video. Honestly I didn't even know if I was doing damage to him or not most of the time, or if there was some invincible dodge I wasn't aware of. Compare him to the piece-of-cake ghost train, and the appropriately difficult final boss, the mad scientist was a pain in my butt. Yeah, that entire game is brutal. For me, MMZ1 was probably the hardest. Fighting Aztec Falcon almost right out of the gate? He's about as difficult as Sigma from any of the first 3 MMX games. --- For myself, it's not so much a particular boss as two particular kind of bosses: scripted fights and fights that test you on skills you haven't needed as a player. Spoiler: Final Fantasy Type-0, spoilered because it's still sort-of new I just beat Final Fantasy Type-0 today, and all of chapter 8 was just miserable. Leading up to the final boss, there are several puzzle or challenge rooms. In one, you can't use physical attacks. Okay, so I'll just use Deuce and --- what do you mean her floating music notes count as a physical attack, and now she's dead because I broke da rulez? And the room with the pillars: "Let the flames guide your destructive tendencies". Oh, clearly this means that of the four pillars in the room, I can destroy any three regenerative at any time I want but the fourth I can only destroy while the decorative flames are in a particular place; or that the same hint indicates I can only actually hurt my enemy while these temporarily-lit candles are presently illuminated. Obviously it means that. I never used any such features before in this entire game, and now I need to learn and apply it in a time-limited setting. That irritates me greatly. And on the scripted boss note, the final boss was similar. I kind of figured it as soon as my first and worst fighter absolutely refused to die and tag out, and when the boss would only deal (damage = currentHP - 1) damage at any time. There's no tension, no risk of failure, only the endless slog of listening to Jack say WAAAAAHHH every four seconds for ten minutes while inching close enough to actually attack. Sigh. The game was good for the first six chapters.
Kirby Planet Robobot the true arena star dream OS. It has so many stages, you end up killing it like 3 or 4 times before you actually win lol