Final Fantasy V
Final Fantasy VI
Final Fantasy VII
Final Fantasy VIII
Final Fantasy IX
Final Fantasy X (over 300 hours of game play)
Final Fantasy X-2
Final Fantasy XII
Final Fantasy XIII (yet to complete - over leveling in grand pulse)
Final Fantasy Tactics
Final Fantasy Chocobo Ranch
Final Fantasy 7 Crisis Core
Final Fantasy 7 Dirge of Cerberus
Final Fantasy Crystal Chronicles
Final Fantasy Crystal Chronicles - Crystal Bearers
Final Fantasy Crystal Chronicles - My Life as a King
I agree with Vooqui, FF7, FF8 and FF10 were my favourite. FF6 had a really good story line but FF10 had so much romance and heart felt moments it was really sad. I even had to kill Yuna about 30 times so that I could pair with Rikku (lol). FF12 had a unique battle system and I really liked it. It got a bit annoying when you had a 4 hour battle with a fiend called Yizamat... that got on my nerves.
As for FF13, I haven't finished it yet. It is the only FF game I haven't sat down and been engrossed in... but it is good. It's also harder to over-level as there are constant caps on your crystal gauge. You also can't visit any towns once you've been there before which is a shame due to the intense graphics.
Also note that Vanille... while cute and rather good looking for a computer character is one of the most annoying people. She changes her voice every couple of minutes.
If your one the edge of thinking of buying it, save your money. Get FF13 versus instead. It's got blood and gore, the first for a FF game. It's also a "darker" game making the storyline a lot more interesting.
PS. Sorry for the essay comment, I just am a huge fan.

Feel free to ask any question and I will answer if you want.