without defining OP's definition of 'best', i will assume (1) unlimited price range, (2) market leading equivalent features and durability, leaving (3) performance.

the short answer for 1/8 nitro buggy: sanwa pgs-xrII

by the most important metric (results) of a subjective question (best), within the above parameters, this is the servo that beat(s) everything else on the market. period.