I think if any Megami Tensei game gets you, it'd probably be Persona 4. And not just because it's bright and colorful. They all have really well done characters, and dark storylines. Most of them are dark in their looks and feel as well. P4 has a lot of fun down-time and character interactions

This one has a lot more Catherine influence, which was a really whacked out experiment in gameplay and art-style, and very much adult. Struggling with death, and marriage, and cheating, and ... wetting the bed in your 30s lol

But I'm no SMT expert or anything. Most of my opinion is just inferred, rather than experienced. I haven't completed any SMT games, sadly. But really enjoyed what I played of P2, P4, Catherine, and Strange Journey. Did not much care for Digital Devil Saga though, but that one was in its own little world. But I think I remember you saying you played that one..