4 with 6 close behind. Both of them had great stories and incredible characters that have stuck with me over time. Rydia is still my favorite character from any video game.
1 and 5 were tons of fun, but didn't have great stories. Of course, that's referring to the NES version of 1. I played the GBA version, but it was so ridiculously easy that it became kind of boring(off topic, but I thought Suikoden 5 was ruined the same way, it could have been a great game)
7-10 were enjoyable but all had their flaws that held them back for me.
I loved the materia system in 7, but the characters didn't really stand out for me like they did for so many.
8's draw system was cool in some ways but bad in others and the characters were rather irritating at times. Squall and Irving in particular.
I loved the equipment system in 9, but the story was junk.
10 was just forgettable for me. I didn't like Tidus or the sphere grid.
I haven't played any FF games newer than that, though I have 12 in my stack of games to play.