I personally loved FFX - my first FF and in my opinion the best. I played FFVII for an hour or so (probable more) and found I just couldn't stand the ATB battle system... It was ok, but not enough to get me hooked, certainly didn't detest it like FFX-2. It might just be because I'm spoiled by not being old enough to play them when they came out, but personally the text in FFVI just left me uninterested in all characters bar Kefka. Kefka was great, Kefka kept me playing for another 10 hours or so. I don't really get all the hate at FFXII though, I haven't played it but it reviewed quite well when it came out, feels a bit weird seeing so much vitriol.
Personally I don't think the series in general was ever *that* great, though it is most people's first experience with a JRPG. FFX's story was amazing - to me. I hadn't ever seen anything like it before, there is no western equivalent that feels remotely similar. All the others then just felt the same, but without me giving a damn for any of the characters, because they weren't the ones I grew to love. I think even if I played FFX-2 first, I would have been enthralled by something that seemed so different to me. I still almost tear up everytime I hear "To Zanarkand", but I have Nobuo Uematsu to thank for that.
I think it just made FF's decline inevitable. We all have our favourite FF (if we like JRPG's at all) and beyond that it seems really hard to get someone to say they felt any other was anywhere near as good, and most of the reasons become nitpicky, like there is some uncanny valley like reaction to playing a game that is really similar, but just not the same.
