FF7 is the favorite for many. Great game. Not sure if I'd say it's my favorite though. 8 was my favorite for a long time, but that may have changed. 9 gets a lot of flack, but I love it. Possibly my current favorite.
8 is very different from the other ones (draw system). 9 is more of a throwback to the previous games (especially the first few). It references alot of things from the older ones (the four elementals from FF1, some references to FF2), even some music from FF7.
If I were to rank them:
9
6
7
8
4
10
5
2
1
12
3