I'd probably have to go with 4, which was the first with a full-on story (not that 2 didn't have a story, but it wasn't as deep or coherent as 4), and the first with summons.

FYI, I think the reason that FF7 sticks out in everyone's mind is that it was the first 3d. The same was true of the first 3d Mario game and the first 3d Zelda game.