I'd say Boost Guardian but I see that's been mentioned several times. Big surprise.
Of games I've played recently, Huff N. Puff from the first Paper Mario deserves a mention even if he's not quite the hardest or cheapest boss ever. Every time he is struck, he generates 1HP minions equal to the damage he took. On his turn, said minions all attack you for low damage and then he eats all the minions, regaining 2HP for each minion eaten.
Do you see the potential for complete suckage here? He can regain 2 HP for every 1 damage taken if you don't work on his minions. Use an all attack? Ha. Most attacks that can hit the flying boss and his flying minions all at once will inflict 3 or more damage and are somewhat costly to use every turn. Yep- hitting all for 3 or more means 3 or more minions to deal with plus his regular attacks (all of which involve mashing A to not die- your right thumb will beg for release)! On a side note, he's the boss of my least favorite Paper Mario Chapter in all 3 games.
In fact this can be considered a blanket hatred for high-HP bosses that can heal themselves faster than you can hurt them.
