Really ANY of the top 4wds are going to be good cars. Some are just a bit more challenging to drive. I have had and or driven a bunch, and I must say the xxx4 g+ is probably my favorite all around. Tons of rear bite, tons of side bite, easy to drive fast. Only issues--needs more steering lock in stock form and can be kind of fragile at times.
Lazer zx5-sp is awesome as well. Stable, lot of rear traction, mucho steering.
501x WCE is a neat car but parts can be hard to come by and I think it can be very challenging to drive lap after lap. Its easy to get one crazy lap but tough to string a bunch of fast laps together.