Actually, FF12 didn't have turn based combat, it was all in real time.
Final Fantasy is reknown for it's exceptional story telling, very long and complicated plots, very deep and believeable characters, and dudes named Cid.
The FF series is unique in that each installment has absolutely nothing to do with the other; both in terms of plot and gameplay (though they do share some commonalities).
FFVI is generally regarded as the best.
FFVII is the most popular
FFX is the most unique
FFXII is the darkest and most political realisitic
FFXI and XIV are MMOs
FFXIII is generally unpopular (as bad as it is, it deserves more credit)
All others are exceptionally high quality RPGs (they ALL are actually)
Also there are spinoff games as well, such as FF Tactics (tie in with FFXII), Choccobo's Dungeon (a roguelike), and many, many others.