Wikipedia corrals some of the suspects here: link I thought I'd put their mugshots up here. Please vote for your favorite in the comments or suggest your own "worst."
(1) Spermidine, the essence of "funky spunk."
cadaverine and putrescine.
(2) Trimethylamine, the essence of "fish taco:"
(3) Butyric Acid, the essence of vomit:
(4) Skatole, an indole & the essence of poop:
(5) Mercaptans--methyl and butyl thiols--the essence of skunk:
(6) Hydrogen sulfide, H2S, the essence of rotten eggs and bad farts:
Runner's up which did not make Wiki's list:
(a) Cacodyl, named from the Greek, meaning "evil-smelling."
(b) Bromine, from Greek βρῶμος, brómos, meaning "stench (of he-goats)"
(c) Cyclohexane thiol: This is technically covered under (5), but to my nose smells like the essence of armpit B.O.