I got sucked in by the first one and couldn't wait for the new Super Nintendo console and the FFII game. FFII was probably the best of them all at the time of it's release. I do like FF7 a lot too though.
2 (AKA 4) was probably my favorite of the classic games. I haven't really been able to stay through the modern ones. The last one I beat was 8. Got pretty far in 9, VERY far in 10, far in 12, and gave up very early on 13. I'm going to try and get back to X since I got the Remastered edition for the PS4.
VII was when I first started playing them, played them all the way through until XIII pretty much screwed everything up (XIII was meh and a struggle to keep myself playing through it, XIII-2 was okay, XIII-3 was crap, played for an hour and stopped as it was so bad).