If you are after a 4wd car everyone is going to have their own opinion. I personally think the 501x WCE is one of the most capable cars available now--although its quite pricey and you most likely will have to stock up on spares. Totally worth it if you can swing it however.
Another awesome ride is the Zx5 sp which probably has the best out of the box setup of any current car--and its not too expensive, only issue is at some tracks parts supply can be an issue, but like the 501x its not too hard to work around--just have to buy more up front.
Also, the XXX4 g+ is a great car that is pretty easy to drive. If you beef it up with the available alloy parts and some tricks its not as fragile as most people think. Parts are available everywhere, reasonably priced and can run with anything out today. Get the front and rear alloy hinge pin braces, steel CVDs or Losi u-joints, and a front one way and you will be good to go.
These three are at the top of my list.