I feel like FFXV will be what FFXIII should have been. Although I liked FFX more than FFXII and FFVII more than FFIX, FFXV will take the short comings of FFXIII and turn it into a more complete game.
People are asking for limit breaks, a world map, more combat interactivity, more characters, whatever. I think XIII was more of the tech demo, and XV will be the real thing. Pretty much for me, XIII was all style, very little substance.
If I had to rank XIII somewhere, it'd probably be one of the lower, even though I enjoyed it. FFX was linear, but it still ranks right behind FFVII so XIII missed out on something, I can't quite put my finger on it, but maybe because it lacks so much.
Good lord so many roman numerals.