The first time I eve played the game, Bolly and Devan were the hardest. After playing the game off-and-on for ten years, I've found that Bubba and Bilsy are the hardest, but it depends a great deal on how the map is set up.

Bubba is fairly unpredictable and can catch you easily in a narrow enough boss room, but he goes down fast.

Bilsy is easy if you stomp to avoid his fireballs, but if the ground was nothing but buttstomp block with a fiery pit below. Having him teleport into you is also a pain.