Final Fantasy Tactics: Advance had multiple of these. But the Assassin makes the top of the list . Ridiculously fast, jump higher than any unit in the game, high attack, evasion and MP, have access to some of the most lethal weapons in the game, can insta-kill you. About as brittle as dry rice krispies, but by the time you get to them they've already murdered several of your team.
The vampire comes close as a second. Ridiculous amounts of HP, can TELEPORT, i.e. height position makes no difference, high MP and attack, and when you kill them they resurrect after 3 turns and return to full HP.
Giant Frostbite spiders in Skyrim. Not really tough, but they're so hideous I always almost shout out loud "GTFO OF MY FACE AAAAAAGGHHHHH!!!!" while fighting them.
captcha: more chocolate. Why yes, yes indeed!