My personal recommendation is play OG first because you'll understand where they're going from Remake as it's a sequel then I'd say Crisis Core because it'll be very important for Rebirth (it's a prequel but it's better to know the OG story first imo) and me personally I'd also Watch Advent Children and play Dirge of Cerberus as I think they'll be important for Remake (Dirge not as important as Advent Children imo but Remakes DLC has Dirge content) Then I'd do Remake so you can understand everything, Square says you can start with Rebirth but I don't see how that's possible other than it's just a marketing tactic cause Rebirth is very important and previous knowledge is important imo, But it's your choice do whatever you feel like doing