FF1: An RPG with incredible replay value! There are just too many different ways to succeed in this game, driven by the party creation at the beginning of the game, and various strategies such as whether or not to class change before going into the volcano. I've gotten tremendous mileage out of this game.
FF4: Most polished Final Fantasy, IMO. Never boring, never slow-paced, never too easy or too difficult, and always entertaining. Fantastic storyline with suspense, political intrigue, betrayal of a friend, love, and yes, the ultimate triumph of good over evil.
FF5: Wonderful job system with lots of synergistic possibilities. "Galuf has died." = most awesome moment in Final Fantasy. NOT the first FF to feature the death of a character, btw -- Tellah dies in almost exactly the same way, sacrificing himself to kill the main villain, because of his granddaughter. Gameplay is here in spades, and the storyline is more character-focused than FF4. Awesome music, even better than FF4's.
FF6: More awesome music! Story is a bit broken with so many characters to follow. Gameplay is neat, but not ultimately interesting, since everyone can do everything. Great first half of the game, rather boring second half. Very cool final dungeon.
FF8: This game's awful, horrid gameplay completely ruined its compelling story. There were truly gripping scenes in this game, but far too much time spent between them drawing magics, waiting for load times, waiting for GFs to finish their summon animation, waiting for this, waiting for that. Ugh. Lack of equipment and the auto-leveling system contributed to this game's utter failure at being fun to play.
Those are all the FF's I've played more than 2 hours of. FF4 and FF5 are tied for the best, then FF6, FF1, and last and certainly least, FF8.