CA CQBR is the best option you could ask for. But if it's too pricey, then a standard CA M4 would be a good option. The gear boxes in the CA M4 and CA CQBR are nearly parrallel in performance these days. The main difference between them is that the CQBR has 7mm ball bearing bushings on it and that makes a big difference in fluidity and ROF.
(PS. If you're thinking of buying the S System, prepare to want to buy a metal receiver.)