3. Leonard Nimoy
Leonard Nimoy has been constantly mentioned in the show Sheldon received his DNA on a napkin, is the subject of the star's restraining order, and there are numerous mentions of his character Spock. Yet in 'The Transporter Malfunction' appears for real, voicing Sheldon's Spock action-figure. In the episode Sheldon and Leonard are given some memorabilia from Penny. The Spock figurine voiced by Nimoy convinces Sheldon to play with the transporter, only for him to break it. He later switches it for Leeonard's causeing the Vulcan to come forward again and chastise him over his dishonesty.. This cameo is works because it really rams home Sheldon's obsession with Spock. Although, to be honest, I'm surprised it has took this long to get Nimoy on the show.