I've only played 7, 8, 10, 10-2, 12, and 13 (and of those, only beaten the ones from 10 on)... and some of the spinoffs. It's a fun series. I tend to prefer more action-y RPGs like Kingdom Hearts or the Tales games but I've really enjoyed almost all of the FF games I've played.
My favourite is probably XIII. I thought the characters were, for the most part, likable. Lightning wasn't what I hoped she would be but is still worthy of my namesake. I loved Vanille and Fang and wish Sazh could have been more than just the comic relief because I really thought he was the deepest and most interesting character--and then essentially ended up as the least-developed one. But the battle system? Oh my god it's so fun. I'm lazy. The auto-battle was godlike. :( And if I wanted to switch off it (which wasn't often), it was still easy to control. I could tailor my party to be how I wanted it with enough work and, well, I really had fun. The only place XIII really suffered in was plot. The plot was cliched and boring, there were really no NPCs of interest, you barely visited any cities, and the only sidequests were battle missions (which, tbh was simple and fine by me but got dull after a while). Also the fact that you literally needed to read the included dictionaries and everything to keep up with all the terms and functions of the world got old too. :/ I was constantly confused and after the years and years of development, I was really hoping for a game that was more complex than "walk in a straight line until chapter 10 and then get lost in a pretty calm lands clone before finding the line and continuing". /rant
Second favourite is probably X-2. Ashamedly. :( I adore its battle system and if I pretend it's not actually related to X, i can pretend Yuna is a brand new, spunky character who wasn't shoved into a totally OOC role for the sake of selling copies! (Also I like fanservice. :D) I only beat it for the first time the other day ('cause I was stuck in the Via Infinito or whatever) and I'm replaying it to get my 100%.
.........apparently I love the FF series enough to go on at length about it. Sorry about that. xD;