It has to be Final Fantasy IX.

No matter how many times I play through it, I love it just as much as my first time playing, if not more so.

A deep, involving story with characters you can't help but make a genuine emotional attachment to, an amazing OST and beautiful, varied landscapes and cities.

For me, Final Fantasy IX rasied the bar not just for Final Fantasy but RPGs in general. The only other non-FF RPG I think I love anywhere near as much is Lost Odyssey, which is an incredibly underrated game.

Besides, even Sakaguchi agrees that FF9 is the best so other arguments are invalid.