No best FF for me as I haven't played quite a number of FFs (I-VI), and there are ongoing FFs as well. I'll just comment a little..

FFVII: First FF to play, it was really memorable. Pretty neat story, character well developed throughout the story and materia system was good. I guess the only complain I have is your party members 'merging' with you...it just bothers me everytime I see that.

FFVIII: Was even more excited when I got the game (shortly after VII). Was blown away by the awesome CGs and it definitely became my favourite. Draw system was annoying at times, but other elements just made up for it. Awesome music as well, one of the best in FF series.

FFIX: I don't know, maybe it's me, but I didn't pay much attention to this game, just swept through it and left it aside. Many people say it's good cause it IS Final Fantasy, but I couldn't care less. Meh, even the main character wasn't appealing (yes, Zidane).

FFX: This one got my attention as well...heck, I saved and waited for 4 years before I could buy my PS2 to play this! And it didn't disappoint me...loved the battle system (changing party members), awesome blitzball mini-game, nice sphere grid, and also a pretty interesting storyline despite the many loopholes. Oh, the music was great as well.

FFX-2: Was interested to see what happens to Yuna and Tidus, but I wasn't too pleased with the outcome. Don't get me wrong, the dress-spheres was a nice addition, but somehow something was missing. Despite being one of the weaker FFs in my list, FFX-2 had the best PC IMO.

FFXII: Although I'm not done with this game yet, but I see a vast improvement in CG and characters' expression, which is good of course. Can't comment too much as I'm only 5 hours into the game, but I can see that the story is promising.