Well I'll chime in with my opinion, excuse me if my choices are not quite as random as some
Best 5 of all time (listed chronologically). I consider them the best simply because they were the dominant 4wd's of their time. They all had/have features that stood out from the rest.
1. Kyosho Optima
2. Schumacher Cat XL/S
3. Yokomo YZ10
4. Losi XX4
5. BJ4
As for the top 5 currently, I would say all of the BJ4 WE style buggies would have to be pretty even - the BJ4 WE, 501X, HB D4, Yokomo etc
I think there's a possibility that the "slim" versions of the BJ4 (Durango, Aero, Atomic) may be even a little better, but until they are actual production cars available to the public, then I think it's not right to include them.