Gonna have to agree with human body odor. That's honestly just the worst. Back in high school, they had DDR sessions after school in on our dorm floor, and some of the people who came never bathed, ever. The smell would linger for hours; you were better off just going to another building for a while.
Goes without saying vomit is putrid as well.
Garbage... eh. My family lives in the suburbs and always complains about how New York City smells like garbage whenever they come to visit, but I honestly don't even notice it anymore. The occasional strong stench of septics gone wrong still gets me, though.