Of the X series, I still have to play X 7 and X 8. I don't have a PS2 but I've heard they're available for the PC, just hard to find.
I don't like the Battle Network games (I tried to get into them but couldn't, despite enjoying a few episodes of the anime), though I did like Network Transmission on the GameCube. It was a platformer, not an RPG like the other ones.
Then I really liked Mega Man X: Command Mission (also on the GameCube, and PS2 probably), which was an RPG, not a platformer like the other ones.
Guess you just have to mix things up a little bit sometimes.
I was going to play the Game Boy games, since I hadn't, especially Mega Man V which is considerably different, but when I heard there was going to be a 15th anniversary (or probably a little later than that) rerelease of the GB games as a compilation on the GBA, I decided to wait for that. And it hasn't come out, likely cancelled by now. >_<