3. Donna & Grace
The sheer juxtaposition of book-smart intellectual, Dr Grace Holloway, and ditsy, street-wise Donna Noble would be immensely entertaining. Indeed, the combination of these two would be nothing shy of brilliant, given that they make up for one another's weaknesses with their incredibly different strengths. As such, it's certain that the Doctor would have his hands full with these two. The dynamic duo of Donna Noble and Grace Holloway would almost certainly usurp the Doctor's authority and could very well see the Doctor taking a back seat compared to the domineering nature of these two feisty redheads. That said, the trio of Grace, Donna and the Doctor would make for an immensely engaging reunion which would certainly please long-term fans and new viewers alike.