Who were the hardest bosses in the game for you? I was surprised at how easy the Guardian of the Worlds was....same with the Final Boss.
I thought the squid and Denny's Nightmare were probably some of the most diffcult, mostly because that nightmare continually poisoned your party. If I had make a decision though, I'd say Gallus was the hardest boss for me personally.
It's been awhile, but if I remember correctly the hardest bosses would include everything from one or more of the nightmare bosses (I had some trouble dealing with the later ones) in addition to the first encounter with the Dark Djinn. During that fight he had some powerful attacks affecting all whilst I probably wasn't leveled up enough to squeak through the fight.