1 was wonderful.
2 I've started around 30 times and never got in to it, I guess it's not great. new systems, didn't work, and they were smart enough to abandon them.
3 was really good. Innovative new systems, but they were all good new systems.
4 was really really good. Dropped the newer system from 3, mechanics were just ok, standard for the time, story was great.
5 was very good. Again they went back to more tried and true systems from 3. Game mechanics were great, story was good enough.
6 they again dropped the mechanics. Mechanics were just ok, I didn't like how every character could learn any spell, made them a little too general. Story was probably the best they've ever come up with ever.
7 story was not bad, mechanics were terrible. I couldn't identify with the major characters very well, I liked some of the side characters much better. Cid highwind was my favorite. I liked Yuffie and Cait Sith.
8 story was not bad, I actually really liked the characters in this game. Mechanics were terrible. Somehow I like this game though better than 7 just because the characters were much better.
9 they revised the mechanics and used a much more standard design. Characters could use omgz armors again! Again I couldn't identify with any of the characters and the story felt pretty generic. I felt it was a low point in the series at the time, but the mechanics carried this game for me, still was kinda fun. Vivi was a good chacter as a supporting character, but he couldn't carry that cast.
10 was not bad. Sphere grid was a terrible idea. Characters weren't 'terrible', I didn't identify with Tidus very much, but I loved Auron. Rikku was endearing. Story was decent enough, but not wonderful.
X-2 is dead to me.
11 I loved :( I played it for 3 years, but it just started feeling too boring, grinding grinding grinding. Killing same endgame stuff. The job system worked well, the story was great, the characters were great, even the NPCs! :P I've actually considered going back now that so much has changed.
12 I liked it personally. license system was terrible. game mechanics other than that weren't too bad. I really liked the hunting system, added a lot of fun things to do on the side. Setting up the gambits was quirky but I didn't think it was too bad, it was a neat way to make the battles real time but still have the characters do basically what you wanted them to do. Story wasn't bad, and I liked all 4 characters. Not sure why these two kids kept tagging along though, never let them in my party with the main characters.
13 first FF game I didn't play. I read lots of previews and it just sounded like something I wouldn't be in to. Also I don't have a PS3, so I held off, but I didn't get it for 360 either after I heard reviews. Just sounded so linear and not much fun. Maybe I'll try it someday at discount.
14. Terrible. What the hell were they thinking. Get some freaking people who play videogames to give advice. They have this habit of trying these new rinky-dink systems with no mind to whether it's actually good or not, sphere grid, license system, well 14 was like they took 30 of these rinky dink ideas and stuck them in one game. Who cares if the left hand it making a super complex crafting system that will require a thriving economy where people can quickly buy and sell things, right hand is going to make no auction house and have a system where you have to spend hours browsing through bazaars instead! hur dur dur