5. Professor Proton (Bob Newhart)
Professor Proton was featured in only one 2013 episode, named The Proton Resurgence, but it's arguably one of the best episodes the show has ever seen. Sheldon is in top fan-boy mode when his childhood television hero is discovered to be available for kids parties and other events... and obviously this results in him hiring Professor Proton for himself and Leonard's amusement. Newhart's role as Professor Proton even scored him his first ever Emmy, awarded for Outstanding Guest Actor in a Comedy Series.