As long as we're counting Demise... Demise. By far.
But just counting Imprisoned fights, oh, I suppose... the third one. Why? Because you do not have to bother with those loathsome toes of his at all. I remember my first couple times fighting him thinking there was no other way but to slice his toes... big problem. The tail, the fact that he walks as fast as you sprint, toes take multiple hits to kill (with eight total), and that the shock wave is massive, made me used to hate him with a passion. But then I figured out you could stand on his head, even while he's on the ground, as long as you Groosenate him first. Yeah, and skydiving onto his head is tons of fun. The only thing I don't really care for is when you take the vents up to the top to get to the Groosenator so Groose can shoot you at him is when you accidentally end up one-hundred and eighty bloody degrees away from the Groosenator, and have to sprint over, while the meter to the left is telling you that you're going to lose miserably at any second.
It's unpleasant.
But it's my favorite of the three fights.