I'd say VII as it brought it mainstream and honestly, I'd have never played the series otherwise. I'd say VI though beats it in some ways, as VI was the first to have a truly wonderful story (arguable- I like IV, but it's storyline wasn't near as interesting). I think VIII could've brought a different direction, and kind of did as X has some of the same feel (though much lighter) and X and X-2 are based around a love story. However I think it's hard to make much of a connection. I think IX could also have hugely changed where the series was going- but that didn't happen.