The Best The Big Bang Theory Guest Stars, Ranked

When The Big Bang Theory began airing on CBS in 2007, it only took three episodes for the first notable guest star to appear. Sara Gilbert, a former Roseanne co-star of Johnny Galecki (who played Leonard Hofstadter in the long-running comedy), played Dr. Leslie Winkle, a fellow scientist and a love interest for Leonard. She went on to become a recurring guest star, along with well-known talents such as Laurie Metcalf, Christine Baranski, Judd Hirsch, and Wil Wheaton.

But since the core four — Leonard, Sheldon Cooper (Jim Parsons), Howard Wolowitz (Simon Helberg), and Rajesh Koothrappali (Kunal Nayyar) — were scientists and sci-fi enthusiasts, the opportunities for more one-off cameos playing to the characters' interests was abundant over the sitcom's 12 seasons. Sometimes comic book legends appeared as themselves, and other times well-known sci-fi actors played a role so totally different from what they were known for. In either case, it was a fun nod to the show's nerdy heart.

To look back on the series, we've picked the top guest stars from The Big Bang Theory and ranked them. While we didn't apply a purely scientific approach to the ranking, we still think Sheldon Cooper would approve. Click through to the gallery below for our full list.