When The Big Bang Theory began airing on CBS in 2007, it only took three episodes for the first notable guest star to appear. A former Roseanne co-star of Johnny Galecki's (who plays Leonard Hofstadter in the long-running comedy), Sara Gilbert 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) -- are scientists and sci-fi enthusiasts, the opportunity for more one-off cameos playing to the characters' interests was abundant over the past 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're known for. In both cases, it was a fun nod to the show's nerdy heart.

With the show's imminent end in mind, we've decided to pick the top guest stars from the series to date and rank them. While we didn't apply a purely scientific approach to the ranking, we still think Sheldon Cooper would approve.

The Big Bang Theory airs Thursdays at 8/7c on CBS.

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