It all depends on what surface your running on. For 4wd it's got to be Xray. I don't think many will disagree with that....though parts are a little harder to come by due to certain people doing certain things.
2wd is a different matter for me. I've tried an xb2 16/17ce for the last year and on high grip it's a very good car,on grass or wet Astro(medium to low grip)couldn't get it to work to keep up with the likes of Kyosho,Yokomo or AE 2wd's.
I've now switched to Losi 22 4.0 for 2wd but not had the chance to run it competitively as yet.
The Losi has both gearbox options in the kit,standup for low grip and laydown for high grip.
One things for sure,your spoilt for choice
Phill