What's the best 1/8 buggy setup ?

The Mamba Max/Lehner combo is nice, but good luck finding a Lehner motor. For something more mainstream, I'd look to an MGM 16018 and Neu 1512/1.5y (1900kv) or 1512/2.5d (2000kv). If you want/need more speed, you could look at the 2600kv 1512. All of these motors run very well on 4S. The first two do very well on 5S and 6S too, although I pretty much limit the battery config to 4S for a buggy. Any more battery on a tight track and the buggy starts handling like crap.