Final Fantasy VI is probably my favorite, just edging out IV. It still had that SNES charm, the score is one of the best in all of Final Fantasy-dom, it has a huge cast and most of the characters are very memorable and endearing, and the World of Ruin is beyond awesome. Also, Kefka.

XII must be my least favorite, because I never bothered to finish it. Never even came close, actually. It was too much like playing an MMO with no other people (like most MMOs, then) and gave me a whole bunch of Phantom Menace vibes, so I just set my controller down and walked away, never looking back.

Your favourite is obviously FF7 :)

Favorite : FFIX

Least Favorite : FFXIII (an abomination of a FF IMO)

favourite: VI, just amazing.  IMO it has the best villain of the series, best graphical style, best story, one of my favourite games ever made.

least favourite: XI, just disliked most things about it.

For the numbered games, I would rank them like this:

VI > IV > IX > V > VII > VIII > XII > III > X > II > XIII > I > XI

My favourite spin off title would probably be Final Fantasy Tactics A2, I would say it is superior to the Playstation game in every aspect bar music and story, in which the later failed miserably 

Fav - FFXII - I liked the setting and how the game was long-lived.

Least fav - FFVIII - from those I've played, it has the worst combat system and worse anything else. Just the music was good IMO.

Final Fantasy VIII is my favourite for the amazing characters and the Junction system, but I don't have a real motivation because also the IX and the X are really good games, but they didn't excite me as the VIII.

Final Fantasy XII is the worst Final Fantasy ever because it has not a deep story.The  characters  can't be compared to the characters of all the other Final Fantasy, and when you find a good Gambit combination you can leave the joypad down and see your party finish the game without your "help". Probably I have this (very) bad idea of the XII because I was reaaaally hyped when the game came out and then I realised that It wasn't the game that I was waiting for.

ff7 my fav because of the storyline/combat and memorable characters.

ff13 my least fav because it is the opposite of my fav.

My favorite is Final Fantasy VI.  It just had a perfect balance of fantasy, technology, a very good cast of characters, probably the most epic story in the series (just imagine if that game was made with today's technology... lots of events in that game would just look stunning!), best score, etc...

My least favorite is Final Fantasy X.  Boring predictable story, worst OST (except for a few memorable themes), most pathetic villains, crappy voice acting, linearity ruined the game for me (FF XIII is as linear if not more linear, but the story isn't as annoying to me and its battle system is actually fun, unlike X where I thought the swapping of characters was annoying at best and the level up system was way too time consuming for my taste).  In fact, I think as little I speak of that game, the better.

FFVI is head and shoulders above any other FF in quality. It does almost everything perfectly from its gameplay (which FF7 then heavily borrowed.....materia system is based on FF6's system), characters, story, music. This is not just the best FF its the best RPG ever.

FFXII is easily the weakest because it had the worst characters, story, music in a FF. But FF3  (DS remake) must be mentioned here too, it was so boring I didn't finish it, but out of the ones I finished FF12....otherwise FF3. Thats not to say FF12 is bad.

P.S. never go to Liverpool by a 3 hour train journey, especially when trains get delayed. Journey time = 8 hours.....time spent there = 2 hours.....result = migraine and £70 loss

My favorite is Final Fantasy X. I liked the characters, the world, atmosphere the battle system is probably my second favorite. And my favorite final fantasy story.. The soundtrack, was kind of scattered but has gems. though my favorite character building systems is still FFV. and then FFX-2(except for this game was too easy)

My least favorite is Final Fantasy XI or Dirge of Cerberous, for obvious reasons... as I don't like MMO's and Dirge handled like rhinosaurius
so mainline I'll say. Probably FF1-3 are worse but I got bored with these and never beat them so
FInal Fantasy IV(not the popular choice I know) out of the ones I've beaten which is FF4-13. I didn't like the world. I did like some of the the characters... and one character is one of my favorites in the whole series. Named Rydia. But, I don't like the battle system that much either... But I don't really enjoy the battle system in that many FF's anyways.... still it's a good game.