Guga's backhand is another candidate but he had a lot of variety. Depends on if we are talking absolutes or just averages. Averages... likely Nadal. Absolutes who knows... Isner's serve?
Extreme Eastern BH, 1x Head IGPMP 12.8 oz, 8.3pts HL with toughgut16l mains and WCSS x's. 1 YTPMP, 1 YTPP, 1 Max 200g, 1 Becker London Tour