Giant Depraved Ones from Demon's Souls.
Honestly, all things being equal, they're not THAT terrible, but you are CONSTANTLY at considerable disadvantage every time you encounter them. Enclosed spaces with problematic camera angles, narrow walkways, tiny islands in a poison swamp that you can't roll in, sometimes multiples, etc. They hit really goddamned hard, my SL 166 character with maxed stamina bar and fully upgraded shield is easily staggered if I block instead of dodge. Plus From loves to play tricks of perspective, so you see them from a distance and think they're normal, only to have them run at you SUPER FUCKING FAST and attempt to clobber you.
God I hate dealing with them. There's an island in the swamp with two or three, I have no idea what's on it because I can't roll in the swamp to dodge, and they rush you long before you reach land.