I never got into the X games much. I played through the original NES series several times, especially Megaman 3 and 6. Megaman 1-6 are all good though (it's been too long to remember specifically what I liked and disliked about each). Megaman 7 and 8 were jarring because of the graphical change.
Megaman 9 and 10 were also great throwbacks. They were also much harder than the originals from what I remember, but MM9 got a lot easier for me since one of the achievements was to beat it three times in 24 hours. It's hard to believe that MM9 is now as old as the original games were when it was released!
Once the games deviated into the X series and Battle Network and Rockman and Bass, I stopped keeping track, partly because I didn't own the new consoles at the time but also because it's hard to keep track of such a long series when the sequels break the numerically sequential names (Tangent: I never noticed that sequel and sequential are related words until I just typed this).
Understandable. Have you played Megaman 11, by the way? I was thinking of picking it up on Steam in a month or two...
Also, my not having played the series mainly comes from my first era of gaming when I had a console and all that being the GameCube era, so almost all of the major Megaman games came out before I had a console or played games. The hardware, and original copies, for that series are starting to get a bit expensive though, so I'll need to stick with emulators. Getting a SNES or NES? Not too bad. Getting a TV compatible with those consoles? A bit harder, at least in my area. Getting the cartridges for the older Megaman games?
Too damn expensive. And by the time I had emulation figured out for the first time or two, my interest was in playing the LoZ games I felt like I "missed out" on, like Link to the Past, the Oracle games, and Minish Cap. Slowly working on my retro backlog, though!
