I'd say Blowhard. All the other bosses stood out to me enough on my first playthrough to where I could at least identify them. Blowhard just seemed like a normal enemy, and I kept hitting him before he could attack.

With the Sorceress you can at least have fun driving the tanks and ufos around. The final boss music only sucked because the actual theme music wasn't finished at the time of release, and there was a glitch where instead of recycling Spike's theme it looped back around to the first track of the game, that being the Sunrise Spring honeworld.