Kuzuhamon and Samudramon in Next Order. The difficulty leap for the final dungeon was insane enough, but then right before the final boss, these two try to stop you. They threw out strong attack one after another and spammed their limit breaks. Even after using the ExE, my back was still against the wall until Samudramon went down. But even Kuzuhamon by herself was no slouch.

Imperialdramon PM from Cyber Sleuth. Kinda iffy about putting this one here. He's the final royal knight, making him the super boss of his game and he's kind of a gimmick boss too, but he was a pain in the ass. It starts off normal enough. Then you get his health under 50% and starts doing a bahamut and charging for seven turns and he's still going all out and you can't hurt him so all you can do is survive. Did I mention the way this game handles difficulty is giving the bosses a lot of speed so they get several turns in a row. Luckily if you survive that onslaught the fight's over.

Ouroboros from Strange Journey. ALL OF MY HATE. MA too, but Ouroboros left a stronger impression.