Series-wise FF is best (meaning that if I picked up a random FF I have a better chance of enjoying it than a random game of another series). However, Suikoden III & V pwn most FF post 7, IV being a horribly executed game, yet a good idea. Grandia II is good, but I've heard bad things about III. Xenosaga wasn't much fun -- it was mostly movie with very little gameplay.

About the only thing FF has going for it right now is consistency. They've never yet had a mainline game that made you think that they were on crack or something similar. With other series, you may get one or two great games, then another 3-4 that just suck. It's a good "brand name".