IMO, the only problem with 5 is the weak castles and the overall easiness. Obviously the ost isn't on par with the first games.
This I can see. It most certainly was easier than the other games, although I honestly liked that. It was a nice breather between the more difficult 4 and 6. Weak castles... I guess I can see. I didn't particularly mind them (save for an instant death trap here and there), but I guess they weren't anything special, really.
Anyways, I played Megaman 6 the other night, and it very easily takes second place as far as the NES Megaman games go. I feel like in a lot of ways, it's the opposite of 5. 5 was content to take what the previous games had done and do it better. 6 tried to do a lot of new things. The Rush adapters were cool (Jetpack especially, when was really fun without being broken), and a lot of the stages did some really neat stuff (cruel, but neat). Mr X Stage 1 gets bonus points for not only having a real cool theme to it (it's the first time I've actually really felt like I was playing a game that takes place in the future) and for being the first "scaling the outer wall" type fortress stage that was really enjoyable (2's and 4's both had some pretty big issues). Overall, I really liked it, even if it got me a bit frustrated at times.
So my overall ranking for the 6 NES games would be 5 > 6 > 3 > 1 > 4 > 2.