I - started the whole thing off. Even in light of the modern titles it's still a very solid and all around game.

IV - did so many things right and polished the template.

VI - showed that the 16-bit era wasn't just all platformers and racing games and that games could move us.

VII - the one that brought in the masses.

XII - the one that wasn't afraid to redefine itself. It may not be to everyone's tastes but it showed that the series wasn't afraid to try and break the mould. A story that was vastly epic in scale and for the most part some of the strongest female characters to date.

Honarary mentions:

IX - Amazing quintessential homage, to me probably the best of the PSone era but it didn't have the defining audience breakthrough that VII did.

X - It was a tough call between this and XII as they both tackle very similar themes. They both attempted to redefine the series by ditching ATB but XII narrowly won out on plot and it's leading female.