Exclusive:

super mario 3d land
mario kart 7
kid icarus uprising
resident evil revelation
paper mario sticker star
new super mario bros 2
layton and the mask of miracles
fire embled
Pushmo
Mario Tennis (yes I have played this one more than mario kart)
Spirit Camera (fun game)
Pilot Wings Resorts (now that it is cheap it kind of worth it)

Exclusive to come this year:

Luigi Mansion 2
Animal Crossing
Mario & Luigi Saga
Mario Golf
Mario vs Kong
Castelvania
Pokemon
Monster Hunter 4


Remakes:
Ocarina Of Time 3d
Star Fox 3d
Monster Hunter 3
Donkey Kong Country Retturn 3d
Tekken 3d

Everybody has mentioned the must haves, here are a few of the 3rd party games you can pick up for it. They are not must haves, but they will be dirt cheap and help pass the time.
Shinobi (old school difficulty)
Ridge Racer ( this is actually quite decent)
Dead or Alive Dimensions
Street Fighter 4 (port, but looks better in 3D)
Tekken Prime 3D
Metal Gear (port,again slightly enhanced 3D graphics)
Sonic Generations ( this is completly different to the console versions)

Mario 3d land
Mario Kart 7
Animal crossing new leaf
Mario & luigi Dream team
Pokemon X & Y
Luigi's Mansion Dark Moon
Castlevania
Lego city undercover 3ds version
fire emblem awakening
kingdom hearts 3d
Monster hunter 4 (eventually)

Super smash bros
E3

3ds XL

Beer x Chips+Weekend= Awesome

Monster Hunter.

That alone is worth 50 games.

You may as well give me the money to punch you in the face.

Easy, easy:

Super Mario 3d Land
Kid Icarus Uprising
Resident Evil Revelations
Mario Kart 7
Professor Layton & The Miracle Mask
Fire Emblem: Awakening
Kingdom Hearts Dream Drop Distance
Legend of Zelda Ocarina of Time (c´mon... it´s a timeless classic)
Theatrhythm: Final Fantasy
Monster Hunter Tri

1. Kid Icarus
2. Mario Kart
3. Fire Emblem 3D (haven't played it yet as it is not out in my country yet, but it's the highest rated in the series, and the GBA Fire Emblems are fantastic)
4. Monster Hunter 3: Ultimate (while on Wii U as well, this series should only be played as a handheld series)
5. Final Fantasy Theatrhythm
6. Mario Land 3D
7. Resident Evil Revelations
8. Ocarina of Time 3D (while old, it adds a few new things to give it a very fresh feeling, and it is better than most games available today).
9. Professor Layton
10. Pushmo/Crashmo

Edit : Nvm lol, I really needa go sleep.

10 must have games for the 3DS?

1. Zelda

2. Zelda 2

3. Zelda: Links Awakening DX

4. Zelda: Ocarina of Time 3D

5. Zelda: Oracle of Ages (to be released)

6. Zelda: Oracle of Seasons (to be released)

7. Zelda: The Minish Cap

8. Zelda: Phantom Hourglass (DS)

9. Zelda: Spirit Tracks (DS)

10. Zelda: Four Swords Adventures

11. Upcoming 3DS exclusive Zelda

To mention a game that nobody ever remembers but is actually very good: Ghost Recon: Shadow Wars.  I just bought it for a tenner since it was a launch game and the game is amazing. Turn based strategy with a lot of content. I think the campaign is around 15 hours long and there are bonus maps.

Well the only must have for 3DS (and the reason I bought one) is Resident Evil Revelations, altough it's coming for consoles now. Also I am waiting for Soul Hackers , if you like old school RPG you may want to take a look.
The other good games (not must have in my list) are:

.Kid Icarus Uprising (Multiplayer is great, single player is good)

.Mario Kart 7 (The multiplayer is good)

.Theatrythm Final Fantasy (If you are a FF fan)

.Virtue's Last Reward (Its also on Vita, but this game is by far my second favorite of 3DS)

.Zelda OoT 3D (Must have if you didn't played the original)

.Kingdom Hearts 3D

.Harvest Moon: A new beggining (If you are a fan of HM buy it now, if not take a look it may seem boring but its addictive as hell, seriously)

And that's it, If I remember more, I will edit this post. Also, while we are on the subject, what Vita exclusive games do you recommend? I have Uncharted, Gravity Rush, Touch my Katamary and Ragnarok and waiting for Soul Sacrifice.