8. Roger (American Dad!)
A bit of a strange choice, perhaps, but I can't help thinking that the Doctor having Roger as a companion would make for a hugely entertaining spectacle. Roger, as a worldly wise alien who is well over 1000 years old, would offer a level of experience that no Doctor's companion has ever had. He is familiar with the intricacies of Area 51, so is aware of the fact that there are other worlds and lifeforms out there (aside from humans and his own kind). He is extremely resourceful, having managed to escape his holding cell in Area 51, and has shown to be brave, kind and caring when the situation requires it (such as when he saved Stan Smith's life when they first met). I imagine the interactions between the eccentric, witty Doctor and the extremely eccentric and sarcastic Roger would be hilarious. And I get the strange feeling that Roger might be attracted to the Doctor, which would add a weird twist to proceedings.