I've only ever used this account for lurking, and this is my first real post, so, I'll give my take on this. Trying to be as unbiased as possible, this is my opinion on which FF's are best (I've been playing the franchise since I was 7, beat FFVII when I was 9):
1. Final Fantasy IX
2. Final Fantasy X
3. Final Fantasy VII
4. Final Fantasy VI
5. Final Fantasies IV and V
6. Final Fantasy XII
7. Final Fantasy III
8. Final Fantasy II
9. Final Fantasy I
100. Final Fantasy VIII (Not deserving of top ten status. Putrid, crass piece of shit example of a Final Fantasy title.)
Now, starting on the two DS titles isn't bad, but, they're radically different in their style and gameplay. FF III offers a lot of customization, and has a varied, entertaining style of play, with a lot of options to keep the game fresh for several play-throughs. Sadly, story takes a bit of a back-seat, though, it's a lot stronger than the original NES version. As for the DS version of IV, story is predominate, with good gameplay to match. The most discernible difference between this version and the original SNES and the GBA port is the difficulty: This one is really fucking hard. Between the two, there is no best way to gauge which is better, but, if you're starting with FF, start with one of the GBA ports, or original SNES versions, as they're more introductory than the DS version of IV. Starting with one of the PS1 games isn't a bad idea either (emulation ftw). IX, in my opinion, has a perfect blend of classic style and what was at the time modern graphics. I'll not only call IX my favorite FF, but, my overall favorite video game. It has a more accessible difficulty with an unforgettable cast, beautiful graphics, and gameplay that never tires.
That's my take, anyway.