every one except for 8 and 10. All were great. And 10 was a little annoying to me, being the main character a something hundred year old prepubescent boy (the characters were my big motivation for not just running around getting to 99). 8 was 7 in a shell of rehash. Squall wasn't the most likable person; even less than Cloud, but it was okay for a while. The two that I don't recommend are really snagged by their sudden spikes of difficulty in unavoidable places, and laboriously far back save points.
9 is the best FF game ever in my opinion. It had great cutscenes; though no voice acting, they still presented a story without any dialogue. It also had a nice mix of new and old and getting rid of a lot of the weird sci-fi things they did with 7 and 8. Plus, nobody anyone cared about died.
I'm running on because this argument has be made with me involved for too long. I recommend 6 and 9 (don't say it) as the two best. 7 is fine, but only the original. The movie was really weird, involving two characters not even connected to the main story (Vincent and Yuffie), and the subsequent sequels were very irritating and just fan-service. Square makes so money off of that because everybody buys it because of that dreaded roman numeral on it.
EDIT: Wait wait spin offs. Ironically because of the recent track record minus XII Final fantasy makes one of the best spin off series in history; FFT. I recommend all four in the set (or the PSP remake of the original) but beware of FFTA2 because its a little slow on the uptake. I like FFTA as the best. Plus the PS1 ones are adorable.