What's the order to play the Hyperdimension Neptunia games (not the rebirth but the og games for ps3)?

I guess you'd play them in order of release, but honestly I'd recommend just playing the Rebirth versions.

Mk2 basically retcons the entirety of the first game, and V takes place in an alternate universe so there's very little actual story continuity between them.

Rebirth 1 is a remake of the first game that makes it actually playable. Rebirth 2 and 3 are more or less just enhanced ports of Mk2 and V.