as far as just the storyline goes DEFINITELY yunalesca. as far as bosses in general go id have to say nemesis.
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
hmm well i probably cant say too much seeing as i just beat the game, and i havent even fought a dark aeon or in the monster arena yet, but a few bosses annoyed the hell outta me, seymore on gagzet mountain, overdrive sin(until i got anima, then he was easy as hell) and i dont know the fiends name, but oh gad i hated this one the most. that plant thing you face in the area right before you meet jecht, where every single encounter is an ambush and its first move is always bad breath...damnit i usualy died before i got a chance to attack!
Most hated boss: Omega Weapon
Most hated Fiend: Great Malboro ( these things suked when you where trying to get to the final boss, Ambush+confusion,poison=GG)