Hmm...
First, I do want to comment on the elitism when it comes to FF games. I see enough hating on FF7 because people "didn't play the superior games on the SNES". I never got that as I have played every single FF game (cept 11) and think FF7 was a worthwhile game.
That said, my fave is FF10. There was room for casual as well as hardcore gameplay. I loved the battle system, that actually made you think and USE ALL your fucking characters. The addition of voice acting really changed the way I felt about its inclusion in subsequent FF games. The story was very unconventional for its time, and though it could've been better, I liked what was there.
My thoughts on the series...
FF1 - Man, this was insanely hard. I hate that 4 fighters had a huge advantage and I hated how the best monks and ninjas basically had you kill those guys off and don't level them up til AFTER you get the promotion. I also hated selecting the same enemy and whiffing the attack instead of attacking something else if it died. Thank God they fixed this in re-releases.
FF2 - I really dug the music, but the gameplay was horrid! Wish I had a copy of that gif about attacking your own party members not being a good idea...Bad Kawazu! Bad!
FF3 - I loved this! Especially when the 'twist' comes. I was blown away by how much land I had to cover after the reveal. I also loved the job system and felt it was implemented extremely well, AND I really wished Square had given this game its due. I remember getting to the end and nearly dying, but then I got the sage and ninja jobs and kicked ass back to the top. Unfortunately, the last boss kicked my ass and I couldn't go back...
FF4 - I played the shit out of this game! One of my top FF games to be sure. I was so hardcore, I managed to get lvl 99 twice, got the silence staff, got the dragon whip and artemis bows, etc etc. Never got the pink tail though...fucking super rare drop! Loved the story of redemption! I do wish that Square didn't fuck with the difficulty so much and that Nintendo wasn't so draconian about its censorship.
FF5 - Seemed like it took a step back in terms of graphics and music, but I'm glad I played and beat it. Boko was cool as hell! Made me wish I had a chocobo. Job system was fantastic, and I only wish Square had also given it the 16bit due it deserved. Man...fucking companies and their tendency not to release games here cause they were 'too hard'.
FF6 - I was blown away by the leap from 4 - 6! The story! The Espers! The magicite! The fucked up rumors of General Leo! I never really got too much into the story and never really understood Kefka, but I loved Cyan, Locke, and the others to death. Fuck Umaro though!
FF7 - This is why I love the series so much. Again, I was just totally and completely floored by the jump! While some argue that 6 was better, I argue every FF game does something better than the others do (except for 2...fucking Kawazu...). I dug the story, Aeris(th), the crazy rumours again, the zany characters, etc etc. I know it did start getting over the top with Cloud's/Sephiroth's sword, but it seemed to fit into the world. Again, while people shit on it, I don't remember any game that attempted to do what FF7 did and did so well. Also got up to lvl 99 and got all the ultimate weapons. I always had Cid and Red in my team...wish Aeris was there though :'(. Materia was awesome too!
FFT - Loved the job system. Hated the VERY nonsensical story. Hated the limited space for characters (you basically had room for 1 or 2 random NPCs if you got every recruitable character). That one on one battle was LAAAAAAAAAAAAAAAAAME!
FF8 - I never understood the appeal and this ranks as the worst for me along with FF2. The story was nonsensical garbage, The characters were unforgettable. The leveling system was pointless, using magic to upgrade your stat was stupid, having spells you could ONLY get from the final boss was retarded. That fucking cowboy sniper...whatever his name is...when he can't go through with his shot, I was so godddamned pissed off at the game...making me get all the way there and then have him flake out. The time travel and witch aspect was just utter drivel along with the Laguna sections too...I wouldn't mind if they never get around to remaking this. Card game was fun though!
FF9 - I really liked the old-school return to roots style. I liked Vivi and Zidane, but the rest were meh, and Kuja was entirely forgettable. I also didn't like how you got skills as well. I remember getting stuck in this one dungeon where you had to do things in a certain order and I just got too frustrated with it. Also, the strategy guide was the stupidest thing in the world! Do this...BUT go to this website link to learn more about it...I didn't get a fucking guide to go look shit up online. That's what Gamefaqs is for...well...that and Cloud vs Sephiroth threads.
FF10 - What can I say? Loved it! Music, story, battle, voice acting. I beat it twice, and sat through it twice with TWO exes (who were my gfs at the time

). Thus, I plopped over 200 hours into this sucker!
FF11 - Never touched it. Though I don't consider it canon or official or whatever you want to call it. However, I will say that any game that requires you to wait in line and waste real life time to get a fucking quest is retarded.
FF12 - I loved Balthier and Basch! Too bad about that stupid Zodiac spear though...Story was pretty good until the time around Matsuno's departure, then it just became a dungeon run over and over again. I lost track of the reason why I was even going to them. Music was pretty meh despite how much I love Sakimoto. I still blame Kawazu!
FF13 - Impressions. It doesn't look that great. I'll still get it, but I'm not very hyped for it. Maybe this will change or perhaps maybe it's me that has changed?
FF CC - I wish they never released this. It felt too...artificial? Forced? I just didn't think it was a good concept overall. Zack is boring, the missions were boring, everything...well...boring...
FF DC - Sheesh...milking this beloved universe for every last drop of that sweet sweet green. I really wish Square had never released that stupid Spirits Within movie...then they wouldn't have to recoup their losses with drivel like this.