from my first playthroughs when i was young and inexperienced:

6: Poltergeist because I couldnt vanish doom him
7: That one that teaches you Trine because I sat there getting whacked trying to learn Trine and even when i did sometimes I wouldn't win and have to fight him again.
8: Edea at the end of the second disk because I grinded my levels up high and sucked at junctioning and had been getting through the game by summoning GF's.
9: Necron and his bulltrout instadeath moves
10: Yunalesca, or maybe thats just ingrained in my memory because of the long unskippable cutscene before the fight.