4. Bob Newhart
"Grab your goggles, put your lab coat on. Here he comes, Professor Proton," and Professor Proton is the brilliant Bob Newhart. In 'The Proton Resurgence' Leonard and Sheldon arrange for a treasured childhood figure TV's Professor Proton to come over to their flat. Prof Proton is treated to showing his basic science experiment to grown men, a rendition of soft kitty while in a hospital bed and seeing Penny, with only the latter registering as a treat. Newhart has great comic timing and showcases it when talking to Leonard, claiming 'you're the genius' after learning about his relationship with Penny. Chuck Lorre had been looking at Newhart to have a cameo in the show for some time, and it looks like his cameo could result in a recurring character. So we could be treated to some more of Professor Proton, and Newhart could get the chance to revisit the role which finally netted him that long-elusive Emmy.