My favorite, even though it isn't the most liked by the general populace, is XII. There was just something about it: Ivalice looked so gorgeous, and the license system allowed for some real customization. The combat system was just amazing to me as well. It was just all-around a great game.
My least favorite would have to be XIII. At first, I was kind of into it. It was new, it was Final Fantasy. But the further I got into it, the less I enjoyed it. Then I got stuck near the end, unable to get past this incredibly powerful mini-boss because I hadn't grinded for ages, and I had no desire to replay it. The combat system was pretty pissy too, so that I didn't like.
Anyone else out there an FF nut?