Have you got a favourite companion performance from Tom's era?
Perhaps it's from a lesser story, but just happens to be a good role?
Some of my favourites:
Sarah Jane - Hand of Fear (perhaps not the best story, but a great role for Lis)
Leela - Talons of Weng Chiang (can't say why, but this is my favourite of Louise's)
Sarah/Harry (Genesis of the Daleks)