who is Sheldon best friend?

who is Sheldon best friend? - The Big Bang Theory

Sheldon's closest friends are Leonard Hofstadter, Howard Wolowitz, and Raj Koothrappali. Sheldon is openly dismissive of Howard and constantly opines that a master's degree in engineering demonstrates. Sheldon considers Leonard his best friend, as they live together and can tolerate each other. Sheldon can only handle having a limited number of friends in his life at a time, but later shows flexibility when he accepts Bernadette and Amy as part of the social group. Penny's having constant fights with Sheldon in the early episodes, they became close friends.