6. D84 (The Robots of Death)
D84 is a secret agent robot. There is absolutely nothing cooler than that. Seriously, the idea of having a robot capable of independent thought and higher reasoning skills is brilliant, and it meshes so well with the Doctor Who universe I'm shocked they didn't get more mileage out of it. I would have loved to see D84 travel around with the Doctor. Think about it, though: all of the advantages of having a robot on board, with none of the annoying logistical problems that K9 introduced on set. It would have been far easier to incorporate him into various storylines, without have to worry about whether or not he would actually function on the sets built for different planets. Yeah, that means missing out on K9, but they could have had D84 during Leela's run on the show, and saved K9 for Romana. That way everyone's happy. I'm just saying, this was a golden opportunity that they squandered. I can only assume they didn't go for it because his robot face is the stuff nightmares are made of.