My favorite is Final Fantasy VI. It just had a perfect balance of fantasy, technology, a very good cast of characters, probably the most epic story in the series (just imagine if that game was made with today's technology... lots of events in that game would just look stunning!), best score, etc...
My least favorite is Final Fantasy X. Boring predictable story, worst OST (except for a few memorable themes), most pathetic villains, crappy voice acting, linearity ruined the game for me (FF XIII is as linear if not more linear, but the story isn't as annoying to me and its battle system is actually fun, unlike X where I thought the swapping of characters was annoying at best and the level up system was way too time consuming for my taste). In fact, I think as little I speak of that game, the better.
