For the world's most stupid boss, I'd have to go with Yu Yevon, from FFX. Seriously, that thing was a piece of piss. Just cast "Reflect" on it, and then attack the fucker. He'll try and heal himself, and restore 9,999 damage to one lucky member of your party every time his turn comes around. Those pagoda things do nothing
For stupidly hard, I'd have to go with Dark Anima (FFX European). it has eight million hit points, and the stupid thing can kill your entire party in three attacks. Even if you have auto-phoenix or something, all it needs to do is use its overdrive attack and its game over.
I am yet to beat it. Afterwards, of course, I'd have to face Penance, which is reputedly much harder.
FFX-2 is way easier. To be fair to Vegnagun, the game's built around the assumption that you'll be roughly level 40 when you face it. Seeing as I make a point of acquiring every skill in ever dressphere as soon as possible, I normally wind up tackling chapter 3 with a level 100 party. At that kind of level, Vegnagun is incredibly fragile, and Shuyin didn't even survive a single "Trigger Happy". That said, Angra Mainyu gave me some trouble, and I still haven't managed to get past cloister 60 in the Via Infinito, or defeat a Great Drake.
Outside of Final Fantasy, for a change - anybody here ever complete Baldur's Gate 2? I have. The final battle against Jon Irenicus is HIDEOUSLY difficult. The dude chucks around disintegration spells in much the same way as lesser beings use magic missile.