My sig explains all. Right now the games I see are Paper Mario: Sticker Star, Super Mario 3D Land, New Super Mario Bros. 2, Legend Of Zelda: Ocarina of Time 3D, LOZ: A Link Between Worlds, Fire Emblem: Awakening, Bravely Default, Resident Evil: Revelations, Metal Gear Solid 3D: Snake Eater, Kid Icarus: Uprising, and Sonic: Lost World.

Edit: He also Star Fox 64 3D and Shovel Knight.

The best games for 3DS are generally considered to be Kid Icarus Uprising, Fire Emblem Awakening and Zelda Link between Worlds.

In my opinion, Kid Icarus and Fire Emblem are much more replayable the Zelda Link Between Worlds. The % of people who have over 100 or 200 hours in those 2 games is surely many times higher.
Kid Icarus has very polarizing controls, if you can manage the controls you will think the game is a masterpiece, but if you hate the controls you may not be able to finish it.
Link between worlds is also great, but you will probably beat it in 15 hours, so if you beat it twice, one in normal mode and one in hero mode (hard mode) that is about 30 hours.

Ocarina of Time is great, and is a must play if you never played Ocarina of time.

Super Mario 3D Land is a very good Mario game, although not a Masterpiece like mainline console Mario generally are.

Resident Evil Revelations is a good resident evil game (feels more like a resident evil game than 5 or 6), the 3D helps to make it more spooky, and it is one of the games that pushed the 3DS the most.

New Super Mario Bros 2 is good, not great.
According to most people who played Bravely Default, the games first half is much better than the second half

The rest of the games aren't that good

Metal Gear Solid 3 snake easter is not a very good port, play the hd version on PS3 VITA or Xbox360.

Play a Sony or Microsoft game.

bravely default

I say Mario Bros first and then the rest! Save Bravely Default till last cause that takes a while to beat

Hmm, I wonder whoes gonna win this since this is gonna be a hard one to judge! Although, Xander has Persona 4 Golden which is longer than all the 3ds games on that list so I think Sony_Girl has this T_T

Oh, and I also have Pokémon Y.

Highly recommend Zero Escape Virtue's last reward. I read a lot of nobels, watch tv shows, movies.... and still Zero Escape has the best story I have seen.
Definitely get Fire Emblem, Kid Icarus and LBW!!
Super Mario 3D World is a platformer masterpiece. Ocarina of time is also amazing and u must play it if u havent already.
Bravey Default is one of the best turn based RPGs.... SMT4 is also pretty awesome.

I would start with Link Between Worlds and Kid Icarus. Both are excellent and take under 20 hours to beat. If you're a lefty you may want to skip Kid Icarus, though.

Every Atlus game.

You aren't a hardcore gamer until you beat every Atlus-hard game on every platform.

Yeesh guys, I'm only keeping this for a month.