http://www.bbc.co.uk/doctorwho/gallery/k9/800/k9_04.jpg
He's the companion that many fans (and certain PS members) would love to have as a companion. Popping up in Season 15 and leaving mid Season 18, he's one of 3 companions to be thought well enough of to get his own series.
But I would argue that Season 16 is K9's Greatest Season.
I'll post my arguments later on, but for now - What Do You Think ?