Hmm, maybe you're right Jormungand, maybe IV is the "definitive Final Fantasy", if not only because it was the first game to give Final Fantasy such an identifiable style. Though I haven't finished any of them, I have played 1-10 and when I think of this fact, yeah IV is the game that I can say that at least V and VI are sort of modeled after, whereas I-III don't really seem closely connected to each other. It makes sense.
I've never really seen anyone say Final Fantasy XII is the best and my friend owns Final Fantasy XII so I guess I'll have to check it out! Now that I think of it, I actually haven't played anything past X-2, so it will be new for me. This is interesting, because you really can hear all sorts of things about Final Fantasy games: VIII sucks, IX is boring, V doesn't have enough story... etc. But then I've talked to people at school about Final Fantasy and they'll tell me that VIII was the best Final Fantasy, and that they've played up to Final Fantasy XIII-2! So you're right, taste in this series is funny, but I suppose that's taste as usual.
Here's what I remember about the Final Fantasy games:
Spoiler:
FFI: Like you said, it's very primitive. I played it very briefly, it would require some patience to get into.
FFII: I didn't like this too much, there was some way you could be penalized in stats? This game is hard.
FFIII: This game has jobs, which is fun... I remember the least about this game, maybe I thought it was kind of run-of-the-mill.
FFIV: Of course, this game seems like one I'd want to finish. I made it around halfway through.
FFV: Maybe I suck at games but I remember this game being extremely difficult, to the point where I could hardly fight some random battles. I had Krile, so I made it pretty far... maybe I'll finish it one day. I made it the furthest in this and FFVI.
FFVI: I made it to Dark World. Maybe it's blasphemous but I couldn't pick a favorite between this and FFIV based on what I played, they're both great games. I like the large cast like you mentioned, and Espers are also fun.
FFVII: This is a classic RPG I guess you can say. Materia is fun, my favorite system up to this point, but that doesn't make it my favorite FF installment. I was somewhere on disc 2.
FFVIII: Drawing magic started out fun, but got really repetitive. Couple it with the fact that I don't want to use my magic because it's junctioned to my stats, and that GFs are really overpowered... kind of blah, not a favorite. I made it to the end of disc 1.
FFIX: I remember little about this game too, like FFIII. I'll guess it's because nothing really stands out about it, it seemed like a really classic RPG... like, really classic. I was somewhere on disc 2.
FFX: Like you mentioned about FFVI, FFX is absurdly easy. When I think about it, pretty much all of them are (except II and V) but this one struck me as boringly so. Something odd about me: I don't play games for story. Yes, not even RPGs. Story is something I expect from a book or a movie but not a game, I expect gameplay from a game and FF10's wasn't that interesting, switching characters was overpowered and the sphere grid was more work than was worth getting a whole sphere level for (congrats, here's an empty node!). And again, Yuna>>>Aeon>>>win, not so interesting. I made it to Guadosalam.
FFX-2: I found this game fun, it had a sort of real-time aspect that was cool. The mission system was neat I guess, and maybe this was one of the games I thought was a little more fun, even if "lighthearted" (I'm sure it gets more "serious" though). I made it somewhere into Chapter 2.
Also, FFVIII probably surpassed FFX in sales because it came immediately after FFVII, so most people probably just bought it outright in anticipation. Seems to make sense. Well, maybe it's time to call my friend and strong-arm FFXII from her...