Rafael Nadal has been most dominant on clay courts. The slow, high-bouncing surface suited his heavy topspin, relentless defense, and ability to construct long rallies until an opening appeared.
His record on clay became one of the defining achievements in tennis. Nadal won 14 French Open titles at Roland-Garros, more than any other player has won at a single Grand Slam tournament. He also collected major clay-court titles in Monte Carlo, Rome, Madrid, and Barcelona.
Nadal’s clay-court dominance was especially striking early in his career. From 2005 to 2007, he won 81 consecutive matches on clay, an Open Era record. His success earned him the nickname “King of Clay.”
This answer does not mean Nadal was weak on other surfaces. He won Grand Slam titles on hard courts and grass, including the Australian Open, Wimbledon, and US Open. However, clay was clearly his signature surface and the one on which his career record was strongest.