Realistically, there can only be one answer to this. It really has to be Bernard Cribbins, who played the part of Wilfred Mott perfectly. He was funny, brilliant and who can forget the poignant way in which he sympathised with the Tenth Doctor as he talked about his impending death - and then discovered that he was the cause of it.
There are some honourable mentions to be handed out: to Kylie Minogue as Astrid Peth in the Voyage of the Damned. The end bit was a bit mawkish and sad, but, let's face it, Kylie couldn't really spare the time to film a whole season, so she had to die really.
I wonder if we'll see Brian Williams, Rory's dad, played by the guy who played Arthur Weasley in Harry Potter, Mark Williams, again. It would be good to see him meet his granddaughter, after all. He was hilarious, it has to be said.