I don't recall him ever actually being called anything but names by Dr. Smith.
He was usually referred to as just "Robot", but officially named Robot B-9.
His name was "Robot". Just to nitpick, in the second season episode "Ghost Planet" he is referred to as "Robot B-9." In every other episode he is addressed merely as "Robot."
In an episode where they travel back in time to just before they launch, there is a crate in Dr. Smith's office. It contains the Robot. On the side there is an acronym, G.U.N.T.H.E.R., which stood for General Utility Non Theorizing Environmental Robot. But as others have said, they always referred to him as Robot. Some people think his name is Robbie, but that is only because he was designed by the same person: Robert Kinoshita. Originally, Robbie was supposed to be the Robinson's Robot, but Irwin Allen could get him for filming the new material added to the pilot footage. Robbie was a busy robot actor back then and made many guest star appearance on various TV shows-including eventually Lost in Space, first as the Robots' adversary, in "War of the Robots" and later on in "Condemned in Space."