Godcat and Akron from the EBF series, just because those fights are absolute BULLSHIT levels of difficulty. Godcat is a 7 stage boss fight, in three of these stages she can instakill you. One stage gives her a superpowered hyper beam which cannot be blocked, and instantly removes all your buffs. The other stages include switching elements, hitting you with four polar opposite elements at once, and huge amounts of accuracy and defense debuffs. Akron is annoying for similar reasons. He switches elements, summons minions, debuffs you, can unleash massive poison attacks, and also can use an attack that prevents you from reviving downed party members.