Over the past three days I have beat 3, 4 and 5. What a varied set of games.
Rockman 3 is quite possibly my favorite one. It is just completely chock full of content. The addition of Rush turned the items from the previous game into a more interesting format and were also a better implementation of items. Everything felt like it fit together. Blues was an interesting mystery that popped up here and again. The extra four stages before Wily's castle were a good challenge and an interesting twist on levels that I had played prior. It all really came together.
Rockman 4 was a huge disappointment.
It really felt like they either rushed this one out or just wanted to distance it from 3. The stages, weapons and bosses were all boring. Instead of the neat stages in 3 there were just two long, dull castles. I really did not enjoy this one.
Rockman 5 was thankfully an improvement over 4. The stages were more varied and interesting. Collecting the letters (and one number) to get Beat was interesting and added in some fun platforming. The Robot Masters themselves were ridiculously easy, however the later Wily stage bosses were not as much of a piece of cake. The Blues "twist" was a nice spoonful of stupid, however. All said, much more fun than 4 was. Much, much more.
I had actually never played 5 before, or at least to completion. I was pleasantly surprised. I just started 6 and it seems to have some decent ideas which 4 sorely lacked and 5 only sort of had. If I ever played this one I don't remember it so I am quite excited to dig in.