If you're in for a looooong game i recommend you Monster Hunter4U/Monster hunter Generations.
It's kind of its own unique genre for 3DS games. You can master up to 14-15 different weapons which will each have its virtues and faulties. You have up to 75 monsters (albetit, recolored pallettes with more moves) to hunt. Each have their own set of unique parts which you use for weapons, armor, etc. Single player can be a bit boring because it lacks any substantial story. But in gameplay it's a great game.
Another A-RPG i can recommend is Kingdom Hearts 3D. But if you've never played it it's story will confuse you so much.
J-RPGS you have PersonaQ, Bravely Default/Second, Dragon Quest, the obvious Pokemon games, Fire Emblem awakeing/fates games.
For platformers Mario 3Dland is a great one, followed by SuperMarioMaker (it's actually way harder than your regular mario game), then NewSuperMarioBros2.
Racing well, I don't think any 3ds beats MarioKart7 in that regard. Nintendo knows how to step up their karting games.
Also Custom Track Grand Prix 7 (mk7 romhack) for 32 "new" tracks (some ported from other consoles, some brand new) just install them with different titleIDs and you'll have 64 tracks at your disposal.
You also have other games like ResidentEvil Revelations, Metroid Prime federation force. its multiplayer campaign is fun, albeit not what fans expected
Visual novel-y you have 999 (DS), Virtues last reward, zero time dilemma trilogy.
Metriodvanias, CaveStory+ (the 2D version) its the improved version of the original game.
The legend of zelda: OcarinaOfTime/MajorasMask remakes from n64, A link between worlds (some sort of a link to the past reimagination) and the multiplayer TriforceHeroes.
Fighting games, i don't think any beat smash bros, though it's not a "serious" fighting game like Street Fighter with the bunch of moves avaliable.
