After playing dozens of RPG's, RTS's, and FPS's, I can safely say that the 2 hardest bosses I ever fought were off of Xenogears. THe first would have to be Id on foot (prolly 'cuz I was using Rico the "I can't hit jack" mongoloid ) and the second would have to be Deus without killing his little orb things (he killed a gear a round... ). Other tough bosses would include Death (name/spelling?) off of FFV and Chaos off of FF1.