Your favorite final boss of any game?

[Big D]

(SPOILER)Jason Chance from Syphon Filter 2. The concept is just so daunting when you fight him the first time: how are you supposed to beat a man in full-body grenade-proof armour, who's wielding an automatic shotgun?

Safer Sephiroth was another foe who was unexpected and intimidating, being the first RPG final boss I'd ever faced. Great character design, memorable music, and neat battle effects (Supernova, for instance). As an RPG beginner, I found him fiendishly difficult too.