This game was definitely one of the best games of this generation. Which FF should I play next? I'm thinking FF XV since its on the PS4 and pretty cheap right now.
Well, FFVII Remake probably is one of the best games to start the series these days or one of the best in the series, period. Other games may have shortcomings in one or another way.
Yes, many loved the original FFVII back then, but it has aged. Still, they changed a lot of story elements in the Remake. So maybe you could go with the original FFVII next to compare and possibly enjoy the differences. But be fully aware: This is a PS 1 game. It will look like one, even as a HD-remaster.
Same goes for FFIX, which may be the best well-rounded of the PS1 era FF games. Good game, but be aware of it's age. Same goes for FFX. Was a PS2 game and a major jump graphically, still looks incredibly dated today. If you get over that, you find still the core of a good game.
More modern games look better and moved away from the turn-based formula to modernize the genre. But may have other shortcomings. FFXIII takes you 20 hours (really), before you can play the full party and all stuff, hence it is often called 20 hour tutorial. It might also be a bit linear.
FFXV is kinda a mess on the story-side. They added a lot story or character development later with DLC. They also tried open world just because, and had difficulties merging a gripping story with the openness.
The games before VII are old 2D turn based games which Square apparently has forgotten about. If you are willing to try, most people like FFVI the most. Square uses it's HD2D (Octopath visuals) now for a Dragon Quest III remake (and possibly I+II down the line). I know some people apparently dislike the HD2D-look, but I think it is awesome and very fitting to remake an original 2D-game, more than trying to transform it into a full-blown 3D-game. So I hope they remake FF I-VI also in HD2D-look.