Actraiser 2 was a series killer. It's not a bad game, but it's hard as hell and omits the hybrid gameplay that made the original game so charming and unique.
Everything that came after the original has been a disappointment.
Space Harrier, Alien Syndrome, and Altered Beast got shafted as well.
It's gotten to the point that I'd rather see fanmade remakes and sequels to Sega's games than anything official.