I've never played 8, 9 , or 10-2. So I won't comment on those.
10 was a lot of fun I thought. The leveling method was a bit different because of the sphere grid, but you sort of got used to it I guess. I got stuck on one boss, and haven't played it since. I have no desire to even beat the game.
Blitzball is all I used to play. That was a fun game.
12 was a a game very similar to FF11, a MMMORPG I used to play. The battle were exactly like 11, but with you controlling every character. It was a fun game as well, but as others have said it fell short with story and character development. I am at the very last battle in that game as well, but I have no desire to beat the game.
I'd say play 9. 10 and 12 were meh at best in my opinion. I think the good FFs stopped after 8, unless 9 is good. With the exception of 11, which is a MMORPG, 10, 10-2, and 12 were sort of disappointments to me. They weren't bad per-se, but they weren't that great either.