Donna Noble was brilliantly payed by Catherine Tate and is my favourite companion of all time!  Donnas first appearance was in 2006 and she travelled with the Tenth Doctor, leaving and then returning. She was a down to earth Temp from Chiswick with an overbearing mother and a wonderful Granddad. Her relationship with the Doctor was strictly platonic (I ain't having none of that nonsense Flyboy) and He described her as being his best friend.  Donnas story arc was one of the best and most tragic.  She was with the Doctor from 2006 to 2010