I quite often have to help people with the computers at work, which means leaning over them to work the mouse or type, and it being a library it tends to attract less 'savoury' customers. There is nothing worse than trying to be pleasant and helpful to someone who has not bathed, washed their clothes or brushed their teeth in a very, very long time. At least if they were dead I wouldn't have to be polite!
The worst one I can think of now is the smell at Tenerife don't know if it's drains or the sulpher/volcanic smell there, also the smell of cooking in Bangkok.