I'd personally start out with Devil Survivor, but there's some things to note if ya do so. Devil Survivor is quite easy but there'll be some missions that'll be random and quite dumb spikes in difficulty. Kishins are great demons, and tyrants are great as well, so it's very possible to keep tyrants and Kishins as your demons until you be upgrade them. In Devil Survivor 1, two people can die if you're not careful and in order to save one, you'll have to go through a good bit of stuff before hand to do. Don't let Keisuke ride his justice craze and keep him away from the murderous gangster, and let Mari kill the Vampire dude, and remember to get her handbag before hand and let Kaido take it to her. They're both extremely good party members, so losing them'll suck big-time.
As for Devil Survivor 2, it's much easier for people to die, so I'd recommend pick your 4-5 favorites and let the rest die, if only for rescuing some of them'll be a real big pain. I'd recommend saving Jungo, Joe and one of either Fumi or Otome for yer picks, but save whoever you want. But yeah, same rules apply for DeSu 2 as for DeSu 1; it's a little harder, but it'll never get too bad as long as you keep up in levels.
Strange Journey is a good one, but make sure you're prepared to use a guide unless you want some grey hairs. The dungeons suck (4 and 5 can go rot in hell) and it's 1st person so it can be pretty jarring of a transition. It's pretty good if you want a challenge, but some of it's challenge is a little unfair *cough* the bosses *cough* but in order to tolerate the game, I'd seriously recommend a guide. Decent challenge along with that and just have a good time with it, as it'll get ya used to getting your butt-kicked and making you try again; something that's almost a tradition in SMT and to some extent Persona.
SMT IV is probably the 2nd best one to start with, as it's not too hard but it's not too easy either and offers a good challenge to newcomers and some casuals of the series. The start of the game is a bit of a crawl, and the Minotaur fight can easily make you have to reset if you get the wrong partner (seriously, if you see Walter as your partner, reset: it'll save you time) and remember to always try to hit a weakness (no duh), but there's a certain OP mechanic. Smirking: it'll make your next attack do loads more damage, it'll make you crit if it's physical, gives a defense boost, makes you basically untouchable and makes you immune to crits (I think). This is done by hitting weaknesses and critting, so abuse it: the bosses will instill the fear of smirking because they'll pound you with it. The bosses are no joke and have multiple press turns because fair fights are overrated. Press turns are the amount of things you can do that turn. Attacking something'll make your press turn go down by 1 and passing makes it a half-press turn, which is the same as a normal press turn except if you try to pass again it'll be fizzled out. Hitting weaknesses or critting only makes your press turn go down by half, basically getting another free action. And these can stack: if say your entire party hit a weakness, then you'll have 4 half-press turns, getting an entire another round of combat. Hitting a null, drain or reflect will cause the attacker's press turns to go down by 2 and the enemy'll smirk. It correlates to the amount of demons you have, so if your entire team except you got wiped, you'll only have 1 action.
And after you've played a few SMT games I recommend you try Persona 3 or 4 out, as the knowledge of previous games makes your time MUCH more enjoyable. I know from personal experience: I tried to play Persona 4 first, but I lost interest and played DeSu 1 and 2, Strange Journey and SMT IV instead. Then I came back and had a blast playing, so I'd recommend saving it for later.
Sorry I had ya read all of this, just kinda wanted to get somethin' off my chest.
Cheers,
Abel