I want to get into the shin megami tensei series but I don't know which to start with. any suggestions? atm i'm thinking shin megami tensei on the snes, persona 3 on ps2, persona 2 on psp or strange journey on ds.
Since the games under the "Shin Megami Tensei" umbrella are pretty broad in themes and such it can be hard to say. I'd definitely recommend holding off on any SMT titles made before the previous generation as, while things like the SNES games and the original Persona for PS1 were considered great games in their day, they've aged somewhat poorly and are kind of clunky and monotonous.
If you want the purest SMT experience go with Shin Megami Tensei: Nocturne on PS2, as this is the newest of the "official" SMT games. The Persona series are spinoffs that have become more popular than the original and are famous for the "social link" and time management elements that have come to define it. Persona 2, which originally released for PS1, is sort of a "bridge" between Persona 1 and Persona 3 in that the gameplay is improved enough to still be playable but it doesn't have the other elements that have defined the series since Persona 3 and 4. Strange Journey is straight-up dungeon crawler and is more like a modern, playable version of the oldest Shin Megami Tensei games.
Of all those you mentioned I like Persona 3 the most, but any are good choices!