I'm not really sure who is considered a "Guest Star," so I just picked one and hope people don't get upset. As of right now, I have to say Bill Nighy as Dr. Black in "Vincent and the Doctor."
For all accounts, this is a bit part as the museum curator-tour guide-person, but he actually becomes a key player at the end with his speech about how Vincent van Gogh is the greatest artist ever - and Vincent is standing right in the middle of the museum listening to his speech. I mean, Tony Curran is wonderful as Vincent van Gogh and his chemistry with Matt Smith and Karen Gillan is fantastic - but there's something about Vincent hearing an expert at a museum in the future talk about what a genius van Gogh was. I loved it.