4. K-9 (1977-1981)
Despite being a much loved and memorable entry into Doctor Who canon, K-9 is included on this list for one specific reason. As a companion, he was really really lame, and kinda useless. While he had all sorts of cool scanners and stuff built in, he couldn't go anywhere! In The Leisure Hive, we see him get fried because he touches water. In Destiny of the Daleks, Romana has to carry him everywhere because the terrain isn't 100% flat. If they wanted to use him, he basically had to stay in TARDIS. Which completely defeats the purpose of him! The TARDIS already has plenty of that stuff, we've seen it used on countless occasions! If K-9 can't be mobile and be used outside the TARDIS, he's essentially useless. Fortunately though, he got a decent appearance in 2006's School Reunion episode, where he kicked butt.