I'd say SMT4 on the 3DS is a good starting point if you wanted to dive in head-first to the mainline series. Otherwise maybe Persona 3 Reloaded for the Persona games.
Edit: A curveball good entry to the series would be Devil Survivor for the Nintendo DS. It's extremely easy to emulate with modern phones, physical copies are not as expensive as the 3DS games (but still expensive) and there is no region lock on DS so you have some lattitude to shop around if you want to use original hardware. In some ways that game does Persona better than the Persona games do, all while having a classic SMT mainline story of law vs chaos.