It was a tough choice, but I feel like the 3DS has to be my favorite, in terms of what it offers, 1st party games, and features overall. It took a while for them, sure, to include some pretty vital/cool features such as folders and themes, but overall, I've always been satisfied with the 3DS lineup.
Of course, the games is where it shines. Surely, their initial lineup of games (and initial console) wasn't amazing, save for Zelda, but over time, they really made it big with the big ones. Smash Bros, Pokemon, Kid Icarus, Animal Crossing, Fire Emblem, Zelda, Kirby, Starfox, Luigi's Mansion, Mario, Mario Kart... all great, memorable games.
I also have a small bias to handhelds. Always convenient to me, personally. Someday I will upgrade my original 3DS to a sleek New Nintendo 3DS, since it seems as if the 3DS line will be living for another couple years because of the arrival of such.