Really nice collection of games. Not all of them are great, as some have definitely aged, but overall they are some of the better NES games that I've played. Plus the collection adds a lot of neat stuff like extra challenges, artwork, character bios, save states, and a rewind feature that I wouldn't have been able to beat the games without. My ranking of the games, from best to worst is 6 > 5 > 2 > 4 > 3 > 1.

I've seen most people say that 2 is the best, and I can see why. It definitely has the best soundtrack by far and has great level design and bosses. But, in my opinion, I think that 5 and 6 are much more refined experiences. They have just as good level design and boss fights, but better menus for the different weapons, a lot more characters like Protoman and Rush, as well as more story overall despite the stories being pretty simple in general. Furthermore, I'd say Megman 6 has the best boss designs and does the Rush power-ups better than any of the previous games, so it's definitely my favorite.