I've played almost all the decent 3DS titles out right now (IE: everything barring obvious shovelware) and here's my two cents.
- Fighters, go with either Street Fighter or DOA Dimensions. Personally I preferred Dead or Alive but they're both magnificent games, either is a worthy addition to your collection. Blazblue is decent enough but its lack of an option to use the circle pad for controls is infuriating. Incidentally any UK gamers reading who DON'T have Street Fighter for their 3DS, it's only £10 brand new on game.co.uk, there's no excuse, get to it.
- RPGs is a dead zone at the moment as the only real RPG out is Tales of the Abyss, and it's so rare that you'd have more luck finding a girl from Essex who doesn't look like a tart. Wait for the inevitable rush of RPGs next year with Bravely Default, etc.
- Flight, you've got either Starfox which is awesome and has great replay value, or Ace Combat which is pretty damn good but perhaps a little lacking in some aspects, one of those 75% games which is a worthwhile purchase but not life changing.
- Racing, you have Mario Kart already, all other concerns are secondary. But Ridge Racer is pretty decent IMHO and it's also cheap as chips.
- Adventure, you've gotta go with Zelda. There's a reason why it's been voted 'best game of all time' more or less every year since its original N64 release.
- Platformers, while you have Mario already, Sonic Generations has pretty good replay value. It's not very long but it encourages you to replay each level in pursuit of the perfect high speed run to gain the S rank to help unlock artwork and etc. Cave Story is also pretty damn good, but since it's cheaper elsewhere you may want to think carefully about that one. Seek if out at a discount.
- Strategy, there's really only Ghost Recon, but it's pretty damn good. It's not quite as charming or as good value as Advance Wars, but it plays just as well. Not much else here, but we've got Fire Emblem on the way soon.
- Shooters, to my knowledge there's pretty much none, barring the rare and strange Dream Trigger, which is...well, it's hard to describe. There's also Resident Evil Mercenaries, which is like one long tutorial until you unlock the last set of missions and get to play for real. It's annoying at times and your inability to erase save data cripples its resale value if you want rid of it, the sound quality is also sub par. But it's worth a look all the same.
- Sports. For American Football we have Madden, but it really does suck. Seriously. It wangs so hard it defies belief. For 'Soccer' (or football as everyone else calls it) you've got FIFA or Pro Evo. FIFA 12 has all the real teams but the AI is alarmingly dense on the standard settings (not a problem at higher difficulties) and there's the curious lack of online play. Pro Evo 2012 has online play and involves a lot of skill, but has few recognisable faces. All depends what you want.
- Misc: PacMan is OK if you pick it up cheap, but don't pay more than £15-20 for it, or whatever currency you use, whatever the cheap equivalent is. Super Pokemon Rumble is wierd but fun in short bursts. Pilotwings is much the same, worth it at a discount but never at full price.
Aside from that it's kinda dead right now. The majority of the bog titles are either coming soon or they're the Mario games you already have. Zelda's a great choice, but most of the games on this list are worthy additions to any collection.