If you plan on gaming it, the tm mk23, if its just for plinking, most any will due.
the tm mk23 has an acceptable indoor/outdoor/close range fps without modification, not to mention the stock silencer really works. other NBB's have higher fps's that require some tweaking/modification to be allowed for indoor/close range.
but if you're just shooting targets sky is the limit.
"You know what the chain of command is? It's the chain I go get and beat you with 'til ya understand who's in ruttin' command here."