My personal favorite is probably FFIX, where there's obviously clearly defined roles, relevant to the plot, but the ability system VeloZer0 mentioned allowed for so much customization within those roles.

I think the safest bet for development though is closer to FFVII and FFXII, where the average non-enthusiast gamer will be able to coast by and not be punished for build mistakes, but there's just enough differentiation on the statistics page to allow veteran players to go hardcore optimization on subsequent and low level playthroughs. They're also two of my favorite games.