Dungeon Siege II: FREAKING MIMICS. Oh, look, a treasure chest I wonder what's insi-- NO, SCREW YOU, IT'S A MONSTER THAT'S LEVEL 45 AND HAS 22000 HP, EL OH EL OH EL
(Actually to be honest mimics in almost every RPG game are a pain. But DSII took it over the top.)
Legend of Dragoon: Lenus. Ugh...