What should I buy?

Out of: Comic Jumper, Limbo, Shadow Complex, 3D Duke Nukem or Castle Crashers.
 
Thanks!

My money is on Shadow Complex or Castle Crashers, but that's just me.

If you are looking for a solo experience, Shadow Complex.   If you want to play with friends, Castle Crashers.   You can't go wrong with either of those. 
 
Limbo is amazing as well, but divisive.  I can't possibly guess if you'll love or hate it.

My money is on Shadow Complex or Castle Crashers, but that's just me.

If you have friends to play with Castle Crashers. Otherwise Shadow Complex or Limbo.

Limbo or Shadow Complex would be at the top for me. 
Then Comic Jumper..

Like with all of these choices it really depends what kind of games you like. Personally I'd choose Limbo.

Shadow Complex or Castle Crashers fo sho.

None of the above, I would save my money and wait for something good to come out.

Shadow Complex or Castle Crashers as virtually everyone has said. 
 
Limbo = Overrated piece of crap that people go nuts for because it's indy and in black and white, despite the fact that you can find a million free games just like that on most flash game websites. 
Comic Jumper = Pretty damn funny but shockingly bad gameplay, the joy you get out of the comedy isn't worth playing through the actual game. 
Duke 3D = Well...doubt I need to put a review for this. But if you like the idea of playing online you can kinda forget it because nobody is playing that game on xbl.

" Like with all of these choices it really depends what kind of games you like. Personally I'd choose Limbo. "

Get Duke Nukem 3D on GOG. It's 5.99 and exactly every computer made in the last 15 years can run it (including Linux, Solaris, BSD and Mac systems), then get one of the multiplayer source ports and join up in the online deathmatches on the platform where people actually play.

Limbo does look awesome though.

I'd go with Shadow Complex.

Shadow Complex.

Comic Jumper is just horrible  dont waste your money on that dog crap  
get   Shadow Complex

Shadow Complex.  If you got 2 or 3 friends to play with then you can't go wrong with Castle Crashers.

I like what ive played of Limbo so far, and you can't go wrong with Shadow Complex in my opinion.