Play em and judge em for yourself.
Personally I've only played a couple of the games, but of them 6 was my favourite. The much applauded 7 wasn't bad by any means either, but I found it got pretty tiresome after a while and frankly I didn't like any of the characters nor the story in general. Still, not bad. The others I've played were all years and years ago, they've since blurred together so that I have no clue which is which though I remember enjoying them at the time. We're talking NES and SNES era here, so make of that what you will, I've no clue of the naming conventions.
The only one I actually hated, which was coincidentally also the last Final Fantasy game I would ever play, was 8. I deplored that game then, and I continue to deplore it now. I tried to like it, oh I did, but how that backfired! My multiple attempts to slog through the game in some vain hopes it would become good only resulted in creating within me a burning hatred which has since then tainted my very perception of Final Fantasy as a series, resulting in me having not touched the series since. I can think of nothing I like about the game save perhaps that it had very good, if wasted, CGI for its time. Beyond that the whole experience is a black stain upon my subconscious which only years of intensive therapy has managed to lessen to the point where I'm capable of subduing the manic depression, flashbacks, nightmares, and urge to kill indiscriminately... until now anyway, thanks for bringing those disgusting memories back into the light.
Back to the shrink I go!
But seriously, play em for yourself. For some mysterious reason many people seem enjoy 8, perhaps you'll be among them so go ahead and try. Ain't my job to judge peoples taste in games after all... that would be Yahtzee's. Lucky bastard!