Personally I had more trouble with Cosmic Wall regular than Hard mode. The extra enemies really helped out.
Originally Posted by MiGfire
I guess another level worth mentioning, was Pyramid Cave on hard mode. Wasn't the hardest, but it was difficult, due to ghosts always bouncing around to mess you up on the key A rank areas.
That was probably the most frustrating for me as well. Having to drop and pick up the key was annoying because you basically had to stop moving to grab the key again without somersaulting, and then throwing the damn thing was bad too because half the time Sonic would throw it downward.