I assume you're young and haven't played many retro games, is that right? i'll suggest emulators and such after mentioning a few 3DS games. Oh, i just noticed you said 2DS. I strongly suggest you try and save for a 3DS instead, i don't think they're much more expensive and the 3D effect is unique to this console, you (sorta) can't emulate it, and we might never get another 3D screen on a console again. Just sayin, i'm from argentina so i'd understand the i can't afford it
Anyways, mario kart 7 is wonderful, i'd say one of the best 3DS games that everyone could/should like. Then you have pokemon, i'd recommend USUM>ORAS>XY in that order. If you like RPGs, bravely default is a very nice one, really captures the feel of the old final fantasy games IMO. Zelda link between worlds is also a wonderful zelda game that captures the classic (link to the past, and similar) zelda feel, while doing something new and looking very pretty. If you like kirby there's a few games to try too, you know what to expect from kirby, the 3DS games are more of the same. You have a few mario party games if you're into minigames and playing in short bursts or with friends. Luigi's mansion is a lovely and unique game, definitely worth playing too.
I could name a few more games but those are enough to get you started i guess. I'll talk about emulation ports and retro games now. You have the entire DS catalog, it's huge! I personally would reccomend Disgaea, it's a great RPG, often overlooked.
Sonic. There's ports for 1,2,CD and reciently even Mania. I suggest you try at least sonic 1 and 2, which have official and easily available ports. There's 3D ports of other classic games like kirby's adventure , outrun and shinobi 3. I'd understand if a younger gamer doesn't like those games (specially outrun) but it's worth trying them just to see them in 3D. You also have ports for DOOM, Quake, cave story and some other games, total classics you should at least try, many also in stereo3D. And then there's emulators for all the retro consoles. No bells and whistles there, no stereo 3D and such, but 3DS can emulate anything from NES (megaman, battletoads and double dragon), SNES (earthbound, chrono trigger), Genesis (sonic, zool, quackshot), GB (pokemon,kirby,zelda,medarot), GBA (sonic advance 2, advance wars, mario kart), Wonderswan (digimon), MasterSystem, NeoGeoPocket, ScummVM (monkey island) and some N64 and PS1.
Emulation on everything but N64 and PS1 is pretty much perfect given the right emulator (some require some slightly complex workarounds like using a DS emulator on 3DS). N64 and PS1 are N3DS only, and even there i doubt the compatibility is great. In that case you'd be better off with a PS vita, where PS1 runs natively and you also can run PSP.
Sorry for the huge wall of text, but when i ask for stuff when buying new consoles i expect something like this, so hopefully it's helpful for you!
