Low-G Man is one of my favorites.
Yes, it's not perfect, but it has some strange appeal to me, and great music for the most part. Had a few interesting gimmicks, like freezing enemies, and jumping entire screens at a time, riding vehicles, walking on ceilings, Weapon pick ups, and a weird aesthetic that meshed well.
Whomp-em was another good one, though possibly offensive and ostensibly just a rip-off of the Mega man formula, it was rather fun.