Elementary
Octorok
Then yeah you'll probably enjoy mainline. Nocturne's a good starting place, but my personal preference is Strange Journey. Surprisingly, Devil Survivor OC is also a good recommendation on that side, though it's still a spinoff. Vengeance should be good when it comes out, but every so often these games expect you have a bit of context to make the experience better...Exploring palaces/dungeons and RPG aspects like battles system are things I most liked. Story, characters and outside activities are cool too, but I liked more the RPG part of game.
It's a bit of an oversimplification, but there's a reason that Persona outsells the rest of the MegaTen games. What most people like is characters and the social elements, instead of post-apocalyptic dungeon crawling with the occasional cutscene.I don't necessarily know about that - I love both SMT and Persona. It's probably more that if what you really love about Persona is the life sim/slice-of-life stuff and not the combat/demons/dungeons, then SMT might not be for you. But if you do like that side of it there's a lot to like in SMT as well.
And I say that as someone who LOVES post-apocalyptic dungeon crawling with the occasional cutscene.