Time for my to break out my recommendations.
+Colors 3D
+Art of Balance TOUCH!
+Mighty Switch Force
+Pushmo (Pullblox)
+3D Classics Kirby's Adventure
+The Legend of Zelda Link's Awakening DX (VC)
+Mario's Picross (VC)
+Gargoyle's Quest (VC)
Colors 3D is simply a fantastic drawing package for the 3DS, I've had a lot of fun sketching and painting with this remarkable painting application, I'm currently working on a nice piece based on a photo I took.
Art of Balance TOUCH is definitely going to be a sleeper hit. A game centred around stacking blocks into a tower and keeping them balanced, this actually is a very creative and challenging puzzler, with many surprising mechanics in it. Check out the video on the European eShop, it's awesome.
Mighty Switch Force is a highly polished puzzle platformed from WayForward, so naturally it has awesome music by Virt, great gameplay, and cute, sexy girls.
Pullblox is one of the best puzzlers I have played in a long time, the simple act of pushing and pulling out walls to ascend to the goal at the top is simple to grasp, yet challenging to conquer. It hardly needs introduction as THE eShop essential.
3D Classics Kirby's Adventure is the classic NES title remade faithfully for the 3DS. With an added 3D effect, which surprisingly only adds so little, the game itself controls as perfectly as the original, in fact better. NES technical limitations are a thing of the past, there's no garbage graphics on screen and the framerate is utterly perfect. This is the definitive version of the game, I recommend it over Nightmare in Dreamland.
Link's Awakening DX remains to be one of the best top down Zelda titles produced, nuff said. If you're a fan of playing around with glitches, there's actually a surprising amount of fun to be had playing around in the realm of the Doghouse Glitch. I've played the game for over 25 hours because of it, and even found a glitch that forces the 3DS to shut down.
Mario's Picross is a simple version of a fan favourite puzzle game called Picross, where you are told the number of panels to be filled in each row and column, and have to figure out which panels to fill in. With some pretty catchy tunes, as well decently challenging puzzles this will whet your appetite for the upcoming 3DS sequel to Picross DS.
Gargoyle's Quest is a platforming game by Capcom, starring Firebrand from the Ghosts 'n Goblin series. Surprisingly challenging with tacked on RPG elements that gradually open up more abilities, it's worth checking out.
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Other titles I feel I should mention but can't recommend for one reason or another.
VVVVVV is waiting to be played on my system memory, I'm sure it will be great.
Dillon's Rolling Western I've only played a little, but I've enjoyed so far. I haven't really gotten into the game like I have Kid Icarus Uprising though.
Super Mario Land 2 is...well, it's too short. You can beat it in two hours.
Pokédex 3D is a neat app, and it's free but hardly essential. Nab it whilst you still can.