SPOILERS!!!!
I don't hate any of the enemies, if I did I probably wouldn't be playing this. I severely disliked a few fights as being too tedious...such as the Cubia Core fights, and the fight with that crazy harp-dude...um..Tarvos. Jerk took way too long to Protect Break. But most of the other fights were good challenges. I hated to fight Maha though....
-Sniff- Beating up on pretty, cute, sexy...erm...

forget I said that >.> <.< Mia was just a tear-jerker for me. AND THEN SHE DIED!!! WAHHHH!!!!! Then again, you get her back..eventually...but Mia was like...the most huggable character for me.
...I seem to have drifted off-topic. Uh....for strongest dislike, enemy-wise, anything with Magical or Physical tolerance.