The absolute worst smell in the world is the smell of death. While I was out hunting 2 months ago, a buddy of mine hit a bobcat that ran in front of him at 1 in the morning. (It was completely accidental, he was cruising and didn't have time to react). But about 4 days later, he decided to skin it and wanted me to help. During these 4 days in the hot 100 degree desert sun, the bobcat had started to develop a unique scent. I never noticed how bad this scent really got until we started skinning.
I was holding its front 2 paws (one was furry and the other had been skinned and was a forearm bone covered in muscle.) while my buddy was skinning the stomach. He accidentally dipped the tip of the knife too far into the stomach cavity and punctured the gut. Out of this came spewing a green slurry of rotten guts and steam which hit me right in the face. Without hesitation, i threw up all over the sand around me and was dry heaving for a good 2 minutes afterwards.
The smell was undescribable. Just a concentration of death and decaying life that penetrated every nook and cranny in my sinuses. The smell was so thick and concentrated that every breath afterwards just seemed to carry the rotten stench deeper and deeper into my body. The very essence of the stench just made me cringe and shiver and continue to dry heave. After literally about 2 minutes of breathing and heaving, the smell started to subside. But I still had to get the green organ goo and fur off of my hands. I wasn't allowed to wash inside because we didn't want the smell in the camper so instead we used about 5 waterbottles to clean the shit off of my hands.
TLDR:
Death and liquified, maggot infested, slimy, green, rotten gut paste.