Very close match between 6 and 7.

Six because it had the best story ever and really good characters.

Seven because it was the first RPG that I played through to the end. I did close to everything in that game except Ruby Weapon, and that was because I raised my own gold chocobo. I got all the enemy skills, got all the limit breaks, W-summon, mimic materia, Knights of the Round. Hmm, now that I think about it, I didn't get the joke weapons for everyone, but that's because I didn't like having to play the minigame in Gold Saucer.

Anyway, once I played through 7 to the end, I went back to 6 with a new understanding of how to play RPG's and realized that 6 was the better game, but since 7 was my first, it holds the spot as my favorite