Nocturne's a great starting point if you liked what you played of IV. It's a similar battle system (without those fucking Smirks, though) and doesn't have any fucked-up DLC. Plus, Fiend battles aren't started by RNG anymore, which is good, because that was a fucking retarded decision. Of all the things to bring back from the original, that was literally the worst. I like the story better of Nocturne better than IV, but saying Nocturne has a "larger focus" on it is incorrect. Basically, 99% of the shit that happens in Nocturne is happening regardless of you being there. Most of the time you're cleaning up a mess, not proactively doing anything. Half the time you're not even really told what's going on until after the fact. It's really only challenging on Hard mode, and even there it's only challenging up until a point where the RNG stops being such a huge factor in fights (in Hard mode, an enemy crit will decide the fight immediately in the early game). Mid and late game you should have all the tools needed to make sure you don't die, outside of a lucky insta-death spell.
Digital Devil Saga is another good starting point. The battle system there is a bit more like a standard JRPG, and arguably has a much more engaging story. But note that the story is in 2-parts; the first game has a cliffhanger ending, so don't go into those games expecting to play one and get a self-contained story or anything. Outside of secret bosses, the games are fairly easy.
Also, Devil Survivor is a great game with a great story. Devil Survivor 2 is a pretty good game with a meh story. The two have no story connectivity, so feel free to just play the first and leave it at that. There's not really any reason to play the sequel, it's a straight downgrade, in my opinion. I'd recommend the Overclocked version of the first game, but most people seem to dislike the extra story content, so take my opinion with a grain of salt.
I can't offer much of an opinion on the first few Persona games. I hated the first one so fucking much that I never got around to playing either of Persona 2. Play Innocent Sin first, and then Eternal Punishment, if you want to play them. I've played 3 and 4, though. If you want a fairly typical JRPG system, and you want to mix it with a dating sim, they're decent.
Also, I like the Devil Summoner Raidou games. They're more action-focused, and it's not for everyone, but they have a simple charm to them that I really appreciated. I'd be hard-pressed to call the first one "fun," (the second is far more entertaining with its gameplay but the story becomes shit) but it kept me engaged nonetheless. So give those a look and see if you're interested in those.
tl;dr I suggest Nocturne, it's my favorite of the entire franchise. Digital Devil Saga is a fantastic choice as well. The original Devil Survivor is great. Take a look at the Raidou games and see if they interest you. No opinion on Persona 2.