A few points to make.
Never played Final Fantasy XI (although I have it) and I still wonder why its part of the series proper because it is online. Not saying its a bad game, just doesn't feel right, on paper at least, to be a Final Fantasy proper game.
X2 is the closest game-play wise Final Fantasy we got since since IX. XII, story-wise is str8 up Final Fantasy, with glaring parts missing (I.E. Gabranth being lame, Vayne never living up to potential). Seriously, if 12 played like X2 with some story fixes, it would've been one of the best games of all time. The through backs to older games are great.
XIII, IMHO, has a great story but takes away a lot of control from the player. Reviews have seen this as not a horrible thing, but I don't understand why they didn't institute the X2 battle system. Its basically the same thing, with more control. It has frustrated me while playing. X2 has the chain combos, the paradigm switch is the dress grid, and the attacks per ATB is basically present. Same system full control, hopefully XV re-evaluates this, as XIII scores with the story and having 2 great characters with the others not being too horrible.
IX is probably the most polarizing game in the series. Ive been told VII is, but IX is basically a love letter to fans of I-VI with a bit of a smirk at VII fans. IX is possible the last overall "real" FF in the sense of gameplay and story, It FEELS, PLAYS, and LOOKS like a FF game. That may not mean its liked by all, but the closest one since is X2. VIII, X, XI, XII, and XIII don't have that the complete package
Lastly, FF2 is not a horrible game, its just my least favorite in the series. II, III, and V lose the benefit of not being played upon their original release and therefore aren't looked upon as fondly. Final Fantasy I was so superior to Dragon Warrior I and II that its ridiculous. If FF2 was released as it was supposed to then, it would've held up better. Instead, it was released after FFIX in the US so its hard to judge it by the standards it should be judged against. It still feels like a FF in its early form, and more like a FF then XIII (not a rip on XIII).
In retrospect, II is my least favorite of a great series. I like VIII more as time goes on. It would've been great to have played V in context to its history, it was my most wanted game for years and it may be one of my favorites if I played it at release, then one of my middle games because I played it years later. VI is my favorite, IV is great (and may have the best story), VII is a lot of things for sure, and one of those is that it really feels like a FF game and is a natural evolution of the series. It needed a fourth party member though, that adjustment to 3 hurt the series IMO in that later games feel limited. I don't have much to say about III as I find it boring with good ideas scattered within.
Even know its not an RPG, I would put Dissidia on the list simply because of what it stands for. Its basically I part 2 (watch the ending and you'll get it) and brings the games together in the only way that can make a little sense. Its repetive, but its oozes with love for the series, something that series has seen once (XII) in the last 10 years.