Overall I'd rather my first game be a pick-up-and-play kind of game, as that's what I'll mainly be using my 3DS for, at least for the time being.
I might be saying this because my 3DS is the one with it pre-installed, but I think that SM3DL may be ideal for this. It's simple, and it's really easy to just jump in, play a level or a few, and put it down when needed. Very conducive to brief sessions. Decently long too, since the game's twice as long as it initially appears. It's a bit easy, but other than that, it's a great game.