Your best bet is to save your $300 and keep the stock airbox....but if you want to buy one anyway, AFE with Proguard 7 filter and S&B (there are more, but I can't remember them at this time). Stay away from the K&N though.
I agree...I've done alot of research on CAI's and have established that the stock one is good...all an aftermarket kit will do is wipe your bank account out and make the whistle a little louder. Keep in mind this is just my opinion and seemed to be the outcome of all the research I've done.
i'm runnin the stock intake on my 400+rwhp 06 LBZ duramax and never have any problems of any sort but if you want a CAI go with the AFE,Volant,S&B or Banks CAI but don't bother going with the banks tuners..