John Barrowman. Does he count as a guest star? I believe so. Anyway, I LOVE the dynamic he brings to the show as Jack Harkness and I LOVE how he and David Tennant became best friends on set. I haven’t seen him in Torchwood, mostly because I’ve heard how angsty that show is and I don’t know if I can handle seeing my baby go through loads of angst. But I’m fervently hoping he returns to Doctor Who soon and Moffat continues Jack’s storyline there (he was originally Moffat’s creation anyway).