I only had played the Final Fantasy VII series xD I had played Final Fantasy VII, Final Fantasy VII: Crisis Core and watched Final Fantasy VII: Advent Children Complete. Dirge of Cerberus is not really needed to understand the story completely but it seems cool to play as Vincent.
I really wanted to play Final Fantasy XIII, but hearing from some of my friends, they say it was boring and kinda a let-down. I'm not sure about Final Fantasy XIII-2 though.
The only Final Fantasy I want to play is Versus XIII and Type-0, I mean that trailer just looks fucken beast! ... even though I dont have a PS3.. =( But I do have the PSP for Type-0, its out already in Japan but I'm waiting till its in english =P
Trailer for Versus XIII
Extended Trailer Versus XIII
Anyways, I would say my favorite is, of course.. Final Fantasy VII. I love Cloud and Sephorith! The storyline of it is just epic, I would actually recommend it! That Omnislash ending was just beast! =3
Its one of the most selling RPG games of all time. And I completely agree with that title. The game is quite good, storyline and gameplay. The graphics are DS graphics but a bit worse.