I have tried all the main FF games expect 1-2 ,8 and nine i'm still currently working on and 10 i haven't really played far. Four i have played up to "metal mountain" on both DS and PSP and i should get back to the PSP port at some point.
So it's early to really rank the games but i do it anyways because at least the 13 sequels add fluff. Almost forgot to mention 13. I haven't actually completed it but i'm like 1-2 hour's from the ending. Last time i tried to complete it i almost had a mental breakdown because of it so i don't know if i ever manage to get through it
Tier 1:FF6,FF7, FF13-2.
Tier 2: Out of the top 3 it's hard for me to say what really deserves the first and what second tier so i'm putting rest on tier 1.
Tier 3:FF12 (deeply flawed but still kinda good)
Tier 4:FF5, Lighting Returns, FF3 DS (forgettable tier)
Tier 5:FF13 and FF15 (awful tier)
FF9 either goes to 3&4 tier. It start's out really strong but after the disc 1 it's mostly pretty meeh. It all depends on the finale really.