I'm another one who's managed to put significant time into just about all of them. I'll even specify which one's I've played.
FF - NES - BRUTAL. Period. If you think about when this was released, nothing was quite like it. You had to play way ahead for battling, especially in dungeons, because even if you leveled on goblins for a while, there were still fights that would gib a max level party out of nowhere. It took experimentation and perserveance to clean it out, and to learn where stuff was. Lots of artifical locking to slow the game down. There is a reason the game is liked, because it a challenge.
FF - GBA port - easy, but a fun romp. Much clearer on direction, and the switch to MP instead of spells per rest made casting far more attractive. However, far, far too easy.
FF2 - GBA port - didn't put much time into it, didn't care for the skill system.
FF3 - DS port - didn't care for the battle sequences. All FFs after 9 suffer this fate, where battling even minions takes too damn long. Quit playing it by the time I hit the 2nd crystal.
FF4 - SNES(US) - Super easy for me to play as a 10 year old. I loved the story, and even though I realize it's pretty hammy now, had some good sequences. Game had a significant exploration portion, but it suffered in the dungeon department, which started getting pretty linear.
FF4 - DS - Difficulty cranked up, and the battles are fairly well done. I don't care for the whole new game+ thing, seems like a mcguffin to get people to replay it over. Best 3d conversion of FF to date.
FF5 - PS2 - honestly haven't put enough time into this one to comment, never owned it.
FF6(3) - SNES(US) - Loved this game to death, still do. Epic plot, combat is extremely fluid, lots of neat treasures hiding everywhere, lots of easter eggs. My personal favorite. Major game bug though, in that Evade didn't work at ALL in the manufactured rom, but most people didn't really notice.
FF7 - PC - Yup, PC port player, played it on my dads laptop that would overheat after 20 minutes of play. Put it on an Ice pack. Overall, I liked the game, especially once I got out of Midgar. Never beat it, mostly because I reached 2nd form Seph, died, and never bothered trying again. Didn't both getting any of the Master Materia, so I basically just played the straight game.
FF8 - PS2 - looked neat, however, didn't put in enough time to make a judgement.
FF9 - PS2 - I feel this is a low point in FFs. While the game is accessible, this is when the design side of things started getting weird. I really couldn't get attached to characters. Combat was AWESOME though, definitely improved on FF7's system. I could tell I was being railroaded though, even with the world map. I was playing under a time crunch on this one, didn't give me a chance to properly explore.
FF10 - PS2 - didn't play much, can't comment on story. Looked pretty, grid system was alright. Didn't like the railroading.
FF11 - PC - yes! I've played this one. Has a great story, and while it is an mmo, it's one of the few that really pushes party play from early on, which is what kicked me off of it. Because of how long it's been running (it's still going!) development continues to improve the game.
FF12 - didn't play this one
FF13 - Railroading hoooo! Battle system was different, but not bad. However, you couldn't choose your fights. You had a set of fights you had to do, every time, until about 25 hours into the game. It's like predetermined, _until_ you reach the end. Hate Hate Hate. Graphically pretty, plot was cliche, but not horrid.
overall - I personally favor gameplay over plot, so things that reward exploration of both the combat system and map make me happier. A good plot is a bonus.
I have a soft spot for FF NES, mostly as a respect of when it was designed and what it was doing. It's still a brutal game, where any fight can take you out if you're not prepared, and if you burn your magic early, it costs you big time. Or running into a pack of 9 cocktrices that petrify your entire party after you just killed off the vampire and realized you had to fight your way back out of the earth dungeon because you were blocked and had no remedy spells. There was no "exit" spell back then to just get out. Or encountering the hall of giants for the first time. Or finding one of several trapped monster chests, with ultra hard monsters. Or crossing the bridge in the sky tower and encountering War Machine (00.001% spawn chance), who just nukes your entire party in one shot if he feels like it, and is tougher than the final boss.
And the final boss himself gets special mention. Because he has all your level 9 spells. And he doesn't have a limit on how often he can cast. So you're basically playing the lottery and racing against time before your own spells run out.