Hydrogen sulfide smells of rotten eggs. That's a rather bad odor.

Hydrogen cyanide has the faint smell of almonds. Although that may not be an unpleasant odor, it's one of the worst smells ever, since moments after smelling it, you get cyanide poisoning and die.

Butryric acid smells of rancid butter.

Some other chemicals with really disgusting odors are sulfur dioxide, methyl mercaptan, phenyl isocyanide, phenyl acetic acid, ammonia, and the majority of alkenes.