Every boss fight gave me high satisfaction, even the most painful ones. I got thrilled each turn wondering if I was prepared enough for each battle (even if my team is overleveled). The immersion is truly one of the strongest points of the game.
The ones that gave me a hard time were:
-Royalant: Kill her minions with fire!
More reason to hate bugs...
-Iwaopeln: Beating all the FOEs before battling this coward was long...
-Alraune: What has Mother Nature done? Beating this giant transgenic weed was as easy as removing a chewing gum stuck on your sneakers è_é
-Golem: Reheal again or I'll turn you into pebbles...
I could mention Etreant but after he got his head paralysed he stood still like a scolded child.
Some bosses left me a huge impression:
FOEs? Some were a pain in the anus, I felt like I was chased by some shady dude in a dark alley... Stalkers, Armoths, Sheltors, Ants, Muckdiles, Ogres and Hunters I hate you (although you give a lot of XP). Now most of them get owned in 1-2 turns ^_^