It depends on your budget. Hyper 7 pcr rtr is a good buggy and usually easy to find parts for. It gets good reviews and I've seen them on the track and they seem to work good. My personal choice for a rtr though is the Svm Crono rtr package.
http://home.earthlink.net/~gbdirect/id9.html It comes with a real good motor, a better controller and servos, better starter box, and it's a more complete race package right from the start. Like I said it's a matter of budget.