My personal recommendations are
4 (listed as 2 on Wii Virtual Console, 4 everywhere else, personal fave, PSP version has this, the sequel, and a new chapter combining them, although the DS port is in 3d, sans sequel)
7 (2nd fave, can get on PS1, a bit buggy, but moddable on PC, and PSN)
5 (3rd fave, available on PS1 packaged with Chrono Trigger, bad load times, and GBA. PSN if you're in Europe)
10 and 10-2 (both are good games, although X-2 is much more lighthearted, but has really great gameplay, and tons of eye candy.)
12 (Real departure from the series, with a giant open world form the get go, and a more down to earth, more political story. Good fun)
13 (regardless of the hate it gets, aside from one whiny little emo kid, the story's really good, with an amazing combat system. I don't know why people hated on the linearity, I loved the game until it suddenly opened up, then the game died for me and I couldn't finish it. Had an absolute blast until that point, though)