9. River Song
A bit unfair, I hear you cry? Well, not really. She's just one example of a time-honoured principle that you can judge someone by the friends they keep. I don't mean to hit on River unnecessarily - I love her to bits, which probably says something about me too...also you, but that's another matter. It's just that The Doctor tends to attract the Rivers of the world and every shade of nutty, cosmic waif and stray known to man. Need I mention Ace - you remember, the companion that used to travel around with a rucksack full of high explosive strapped to her back? (Oh, I just have - never mind). Let's face it, River is as mad as a box of frogs. Kill Hitler? O.K., if you were involved in one of the six failed attempts to do just that my unreserved admiration goes out to you, but at least you had a good reason. Hers was simply for the fun of it and to make some point or other that could have been explained in much less extreme ways. The list of dangerously unbalanced oddballs that The Doctor has involved himself with extends ever onward. He has rated The Master as someone tipped over the edge into madness simply for looking into the Eye of Harmony, yet also wanted to care for the warped megalomaniac on the tenuous basis that he was the last other example of a surviving Time Lord than himself. Also, how many times was he prepared to believe the best in Vislor Turlough, willing partner in the Black Guardian's evil machinations? Alright, I accept in the end that Turlough had a logical reason for being the way he was, but it was almost as if The Doctor enjoyed the danger that he brought and you need look no further than that to question his sanity.