I'd honestly have to toss it up between Ten and Tactics. Ten gave me, from an RPG, everything I wanted as a wee lad. I remember popping the disc in, watching the opening AMV, and thinking that this was the future. No other RPG has done that for me. Even the previews for thirteen hold up to that standard.
As for Tactics, the play through itself is a bit fuzzy for me, mainly due to the 200 or so hours i put in to it. Though when I look back on it, even though the system took a bit of getting used to, it was the easiest to get hooked on, in my personal opinion. All those fights that I didn't think I could win at first glance, now seem like child's play. All the strategy I put in to my short term and long term goals for the game, that gave me a sense of accomplishment that very little games can rival. However, Mimes were just a waste of my time.