MEGA MAN 5 / Capcom / NES
Seriously man, I have a hard time thinking of anything to say about Mega Man 4-6 beyond, "Meh, they're not bad, but not nearly as good as the second and third ones".
Mega Man 5? Slightly better level design than Mega Man 4, though there are still some annoying and lazy ones (hi Wave Man). Slightly less silly bosses. One good gimmick, in Gravity Man's level that continually flips you from the floor to the ceiling, though it's pretty short. The music is mediocre, but at least you can actually hear it now, as charging the Mega Buster doesn't block out half the sound channels.
If you somehow wound up in a bizarre situation where you had to pick one game out of Mega Man 4-6 to play, I'd pick this one. It's the best of the worst, but has nothing on Mega Man 2 or 3.
* Gameplay Video