I think every Final Fantasy should be judged independently from the other ones. That's why they're cool for different reasons.
I like VII because the character design was rather cool, the battles were pretty fun (For the time), and the sidequests felt rewarding. The main problem I have with the game, however, is that the characters become cardboard cutouts of their disc one selves by the time Cloud has that lifestream thing happen. The whole "real Nibelheim scene" thing was also retarded. Cloud being a clone, albeit cliche, was much cooler than him beating Sephiroth by just throwing his sword out of his stomach, then later having a long (but still easy) boss fight with him. However, it was made up for by how fun the game really was in the longrun. Especially with collecting the final weapons.
I don't like VIII as much as the others, I admit, but it was still fun to just walk around and get into random battles on disc 3. GFs were fun to watch, too. The cast also was actually.. likeable as characters. Of course, Squall suffers from the same disease ALL main characters suffer from (The whole cardboard cutout thing), but the end of the game just feels like a long somewhat entertaining sidequest anyway. My gripe with this one is the weapon system, which somehow irritated me more than the junction system.
IX was pretty neat. I lost my original memory card some years ago, so I don't play it anymore, but it was still cool. The boss fights sometimes were unbearably annoying, the characters didn't really have much chemistry, and Zidane is Zidane, but hey, the trance thing was really cool. I still played it and loved it anyway. It also had, at the time, my favorite sidequests. I remember I was excited to do them. That was pretty much the highlight of the game for me.
X was a bit of a rocky place. I didn't get it until MUCH later, and I played X-2 first, and really, it was because I didn't care. And still, I don't really play it much anymore. It was amusing playing through it, but I didn't really like the amount of grind the sphere system had me doing. It was like I would start level grinding, and I'd be like "ooo i'm a bad mofo now" and I'd go to the NEXT ROOM and get slayed. Then it was mooooore griiiinding, until I went to the next room again and got whupped. I have a tendency to be underleveled on FF games, but that was just ridiculous. At least Aeons were cool though.
X-2 was really fun until the end. The battles looked terrific, the dresspheres were amusing, and it was overall an entertaining game. Sure, they looked slutty, but at least they weren't boring. They had personalities! (something I see gamers seem to hate in their characters) I never found myself too frustrated or bored to play it during its run as current game, and nothing about the system made me mad. Its plot was braindead, but I think it was.. supposed to be. So no problems there. I just wish they didn't do that thing that is always on RPGs where you get to a point where every monster is either a cakewalk or instant death. It's really bad on this one (as well as VII and XII)
XII is my current favorite. The plot might've been a bit lame, the voice acting was superham, but those two things are tolerable if you have a sense of humor. Gambits were awesome, even if I did only set three at a time, and sure boss fights weren't all that hard, but the hunts were really fun, and just going to some map and wandering around attacking stuff was fun too, which made level grinding not so bad. It was getting tiresome having characters just stand in a line, because well it isn't entertaining after a handful of game with it. It also seems silly after a while. I was excited when I saw the characters walking on their own and no random encounters. It made it less frustrating to try to go from A-to-B without Zubat coming out and pecking at you for 2 HP each hit, and escaping won't work. I was glad that was over. It was great having freedom in battles. It's really a step in the right direction, a direction where JRPGs can be fun and not just entertaining.