Mega Man 2:
Considered by many to be the best Mega Man game, Mega Man 2 had so many improvements over the original that extended the Replay Value. Better stage layouts, improved graphics, Passwords, items, more weapons, etc. This is also a good one to start off with, as it is not too Easy or too difficult.
Mega Man 5:
My favorite in the entire series, because it introduces a new character and item that are actually pretty useful. Great level design, improved graphics and other cool features that make this a definite must have.
Mega Man X:
I list this one because it was the beginning of a new, high-quality, Mega Man series. Not only that, but it is a game that has different characters, plot, and alot more gameplay the original series offered than future X games, which I like.
Rockman & Forte (Mega Man & Bass):
Currently, the last Mega Man title in the original series. Believe it or not, this was released on the Super Nintendo in 1998 in Japan, the same year Mega Man X4 was released in North America. This uses features from Mega Man 7, Mega Man 8, and Mega Man X4. One of the most impressive feature of this game is that the quality of the graphics are nearly as good as Mega Man 8. Basically, this is a high-quality, entertaining title. This is available on GameBoy Advance in English as Mega Man & Bass.
Mega Man 64/Mega Man Legends (N64 & PS1):
Why do I list both of these titles? They're the same game, with different titles, but both versions have a few features unique to them. For example, Mega Man Legends has CD Samples, and Mega Man 64 doesn't, but Mega Man 64 has something that extends the quality of the games' Plot. If one feature is missing in one version, there is always something that the other version has to make up for the missing feature. So, I just recommend getting the version for the system you have. If you have both, it doesn't matter. They're both quality games, so pick the game for the system you like best.