A lot of Louis Theroux ones stick in the mind, for different reasons. The one where he goes to a brothel in the USA, makes me feel so, so sad for the women there who do their best to pretend they enjoy it but clearly don't. And the one where he meets porn stars, the young lad who has to do male porn looks so upset about it... Then on the opposite side of LT docs, there's the classic UFO/alien hunter one which will make me laugh forever.
Then on Channel 4 there was Murder In The Car Park which is truly an eye opener about the UK press and the police force.