I really liked the Moscow mission where you have to find your mark among a room of possible targets, but you only had sketchy details on him - that he was left/right-handed, smoked cigars or a pipe, wore glasses or not, etc. But as good as the hits themselves were, I could rarely get away clean afterward - the guards seemed inhumanly good at seeing through my disguises, especially when the alarm went out.
Ah, that mission was where I found out that the Americans use a different floor numbering system from the British. "He's in a room on the second floor!" they say, so I go up in the building and look into the second floor window, only there's nobody there.
Go down to the first floor, and he's in there.