I've played all the NES megaman games. That'd be 1 to 6, I think. My favorite is definitely megaman 2! Used to play it all the time with my brother when I was younger.

I remember I always beat em in this order:
Freeze Man
Wood Man
Air Man
Metal Man
Heat Man
Quick Man
Bobble Man

and Crash Man somewhere in there. Now I always beat Metal Man first.