Hard court has always been the toughest surface to win because most of the players on the tour are adept to this surface. But between grass and clay, I believe grass display higher quality tennis because a player can impose every of his arsenals on court. Grass rewards big serve, volley, touch, slices and approach shots on full display. Grass offers more all-court game makes it difficult to win, and prone for upset. Unlike clay, there's no place to play safe tennis, no endless rally, a player is pressed to end the points instead of waiting for an opponent to hit an error. Grass also rewards for great serve returner unlike clay big serve gets nullify. Basically, every shots are more effective on grass(except for topspin), and with more variety and tactics, grass has greater competition than clay. It takes more talent to win on grass than on clay.