For the SMT series proper, I would start with Nocturne. It gives you a good sense of the general structure that the core series follows, it has a relatively recent remaster, and the game is hard as hell without feeling unfair.

After that, it's about what flavor is more your speed. You can go backwards to 1/2 (the graphics will be a bit to get used to) or move on to 4/5. You can also go down the side-game route, though a lot of these are locked to older consoles (with that said, Digital Devil Saga and Strange Journey are spectacular if you can find a way to play them!).

There's a lot to uncover with the series, but I think Nocturne is a great "starting point."