My personal favorite is 3/6 but it was the first one I played. Two of my best friends also play the series, and each of them prefer the one they played first (7 and 8, respectively).
Six, in my opinion, blows 7 out of the water every step of the way. Sephiroth is whiny and emo. Kefka, without spoiling anything, IS A MANIAC. And the characters in 6 are much more fun, and 7s story is really just a rehash of 6.