I gotta play these. I don't think I ever played the SNES nor Playstation games, but god I loved Mega Man 2 and Mega Man 3 on NES.
Along with others here, I definitely recommend playing Mega Man X if you liked the earlier NES games. There are some differences, and I think the two that will stand out the most at first are the inclusion of Heart Tanks. In Mega Man, you have your life meter and it never really changes. You can refill it with E tanks (in MM2+), but that's really all you do. In Mega Man X, you start with a lower amount of health (it's very easy to see when compared to the special ability bar), and each Heart Tank will increase your maximum health. These Heart Tanks are usually hidden and may require a suit power-up (one requires an incredibly long jump using the boots upgrade) or a boss ability to get (e.g. you use Boomerang Kawanger's boomerang to grab one of them).
Speaking of suit power-up, that's another big difference in X. You start off as the titular Blue Bomber, but a you find suit upgrades, you'll don a white armor. If I remember correctly, there's an upgrade for chest, legs, arms, and helm. There's even a fun, hidden secret upgrade that requires... I think all suit and Heart Tank upgrades. The first suit upgrade that you'll likely find are the legs, because it's quite literally in the way (in Ice Penguin's stage), so you can't miss it. Some of the others are a little bit harder to find!