Ok, the toughest girl boss battle would be... Lenus, from "Legend of Dragoon" for the PS1. I still havent't whooped her wingly butt yet!
Toughest guy boss, Vergil from DMC3. I sware, i just beat him this week, and it took me 8 TRYS! 8 FU**ING TRYS! God, i hate his attacks!
But the hardest creature boss, well that goes too...Ultima Wepon of FF8, he's very tough, and i still havent whooped his ass, i'm so weak!