Frustrating..? Oh man, I can't think of a clear contender off the top of my head so here's a few.
Any boss in Tales of Graces.
Bane in Batman Arkham Origins. Mainly 'cause he cheats the darn rules. Why can I jump over Killer Croc but not Bane? Not to mention the infinite number of goons.
Bed of Chaos, Dark Souls.
Elizabeth from Persona 3. This is another cheater. Also a terribly designed boss fight. Let's have a character whose the strongest in the game, remove your party so you only have one guy, have her be immune to everything but one attack type at a time, have her completely heal when she's almost dead, let her attack you more then once per turn, and, my favorite, have her insta-kill you if you work out a party that completely counters her moveset.
Hardest in terms of a legit boss that doesn't cheat you... Uh, Vergil in the original Devil May Cry 3, back before they changed it to reduce the difficulty, was a son of a *****. Fun though.
There's probably betters ones, but that's what I got right now.