I am currently playing through IV Apocalypse and am hooked. It is my first entry in the series. My question is:  Are the other titles for 3ds also worth getting into or is Apocalypse such an improvement that if I go back to the older titles I may end up disappointed? 

Shin Megami Tensei IV is definitely worth it. The Devil Survivor games are also good from what I've heard, but keep in mind that the gameplay's different. Soul Hackers is a bit older school, but with some adjusting you should be able to get into it.

Everything by atlus.... is worth atleast trying.

Apocalypse is a bit easier and lighter in tone than traditional SMT, but I still would recommend trying IV to understand the setting.

Devil Survivor 1 and 2 are fantastic, and DS2 Record Breaker is my favorite entry in the SMT series (including the Persona franchise). These games are also a bit lighter in tone, though still have some heavy moments. Gameplay is a bit different though.

I suppose it depends which aspect of the game appeals to you. Do you like the macabre tone? The story? Gameplay?

I like the tone, the music, the story and gameplay. Everything about it has me hooked. I still consider myself a newb to JRPGs though, only starting to play them about 3 years ago. 

So with that said, if you could recommend one other SMT game for 3DS, what would it be? 

Thank you all for your responses :)

IV is amazing but be ready to get lost in Tokyo, and not in a good way.

Still an amazing game, one of the best JRPGs of the 3DS.

ALSO get ready for Strange Journey Redux, that's a must play.

Everything made by atlus is worth trying, if you got hooked on SMT4. If it was a persona fan I'd be causios but not SMT.

4 is great, definitely recommend it. Soul hackers is also fun but it shows it's age since it's basically a straight port of a Saturn jrpg

In that case, I'd recommend SMT: IV. It's either on par or better than IV:A in all those categories except gameplay, though I suppose story is subjective.

I also highly recommend SMT: Devil Survivor 2 Record Breaker...it's just quite a bit different than IV and IV:A, so there's no guarantee you'll like it as much I did.

Eh...there's social links and characters arcs, but other than that, I'd say Devil Survivor stands on its own pretty well. Difficutly-wise I personally found 2 more satisfying...and I enjoyed the Septentrione fights a lot more than the Bels. Also the Tico fight...*swallows hard*

2 had more of a sci-fi tone though, as opposed to the more macabre tone of the mainline games, which is why I don't know if OP will enjoy it just by virtue of liking IV:Apocalypse. Still my favorite SMT though.