bukimi_heishi

Disgaea games are fun, although I get overwhelmed which how much there is to do that I never finish them. I've played 4 and 5 and as far as I can tell, they're all unrelated. 

I've never actually played Shin Megami Tensei myself, but I've watched my college roommates get each ending for Persona 3 and 4, so I know what happens in each. I've also played the Street Fighter spinoff version, if that counts haha. P4 is definitely the most famous one, but again, I don't really think they're necessarily related. From what I've seen, they're a cool mix of RPG styled combat, monster collection, and dating-sim. I'm looking to get P5 when it comes out in September.

Seemingly random, but Digimon Cyber Sleuth for PS4 is also heavily inspired by P4 in terms of combat, story motifs, and overall gameplay feel. If you end up liking Persona, you may be interested in that as well. It's completely unrelated to the rest of the franchise, so no background knowledge is needed to appreciate the game.

I don't know anything about Disgaea but for SMT Nocturne is my personal favorite. I highly recommend it if you want to play something challenging and you're okay with a lot of grinding. It can get really difficult if you're going for the true demon ending and/or play on a higher difficulty. 

Digital Devil Saga is probably my favorite of the spin offs/non main series games. If you want to try something less intimidating I think DDS would be a good starting point aside from the Persona series but I think starting off with Persona 3 or 4 would be best if you're unfamiliar with SMT.