I am sure I will get ninja'd but I will examine this question in detail. As I too, am a SMT enthusiast.
Currently there are 6 SMT games for the 3DS, with a 7th on its way this summer. Those would be as follows:
Traditional turn based:
Shin Megami Tensei: IV and
Shin Megami Tensei: IV Apox - I would suggest starting with these two. The first (SMTIV) is the newest installment in the mainline SMT series. It is a 3rd person adventure RPG with turnbased battles and a heavy emphasis on monster collection and fusion. The difficulty can be scaled to however you like to play. There are 4 endings and tons of side missions to consume your time. SMTIV:A is technically not a sequel per say, but another game featuring a different perspective to the concurrent story of IV. Apox gives you 4 more endings to the overall story that spans IV and IV:A.
Tactics:
Devil Survivor Overclocked and
Devil Survivor Break Record - These are my personal favorites on the 3DS. Both are enhanced ports from the DS with loads of new content. BR has a completely new second story that lengthens the game a good 30 more hours. The Devil Survivor series focuses more on story and turn based tactical combat. You usually are in control of up to 4 teams consisting of a human and two flanking demons. When you and an enemy team get close enough to fight, the battle shifts to a single round of traditional turn based battle. You and the enemy get 1 action each with criticals and other special actions scoring you a chance to gain an action in the extra turn phase (does not stack). There are lots of demons to collect and fuse as well as being highly customizable. I think the story and setting are the coolest of all the 3DS SMT games to play.
FP Dungeon Crawlers:
Soul Hackers - Ok this one is much like SMTIV but the setting is quite different. It's a First Person crawler with balls to the wall difficulty. Your demons do not level up so you must rely on fusion and demon collecting. The original was a region locked Sega Saturn game later ported to the Playstation, but never localized for us. The graphics and gameplay are largely unchanged. ATLUS was very smart with their update. While optomizing the game for the 3DS, they added in game "hacks" that essentially act as cheats you can collect and enable. The 3D adds (quite literally) a layer of depth to the immersion this game offers. It feels old, sounds old, and plays kinda old, but all in a more nostalgic way rather than clunky. You can play the game at whatever level of technical skill you like.
Persona Q - Basically, another Etrian first person RPG but with non canon spinoff stories featuring the characters from Persona 3 and 4.
TB Released:
Strange Journey DEEP: Another first person dungeon crawler, but more along the lines of a main line SMT game.
I hope you find something you like