Probably an impossible question since nobody has smelled every odor in the world I wouldn't think anyways. But worst odor I have smelled so far is raw sewage.

Road kill in the summer when you drive past, wind up the windows we have a lot of it in Australia unfortunately

fungating cancer tumours, you just can't describe it, and you will never forget it.

the worst i believe i have ever smeelled would be either rotten eggs or baby poop

Really bad smelling farts !

mercaptan (sulphur compound) containing chemicals smell really awful (one is placed in gas so you know when there is a leak is Australia). Generally any compound containing sulphur is noxious , as are chemicals emitted by the body when dying or dead (putricene is one). I work with aroma chemicals, and some are just so foul and so pungent, even opening a bottle to get 1 drop out will fill a room with stink for hours even though the room has industrial ventilation running all the time.

Methane generation from human, animal, and agricultural wastes.

Old outback drop loos! You have to take a deep breath, race in, do what you gotta and get the heck outa there before you take another breath!!!!

The worst odor I have smelled is a skunk

the worst i know is rotten eggs. honestly i didnt eat eggs for ages after and the thought is making ne gag now (urrrrrrrrrrrrgh)

This is just my opinion but somebody at my old office routinely made microwave popcorn and burned it every morning around 9 am. The smell of burnt microwave popcorn lingering around the office all day from morning on is just HORRIFIC

death.

My feet prior to having a bath.

butyric acid, trust me its horrible, smells like really strong vomit

sick wen someone has had garlic of my best mates B.O. he fucking stinks

Sewage!On a hot day!

There's actually a scent produced in a laboratory which combines all the worst aspects of the worst smells (Dead things, vomit, shit etc) into one very potent and apparently extremely nauseating smell. I saw it on Discovery channel and the guy who smelt it was actually very distraught after getting a whiff. They were developing it for crowd control.

A McDonald's store.

isonitriles They have been called the Godzilla of odors. One webpage describes them as "Horrible. Extremely distressing" "You can't believe how awful they smell," Turin said. "They make you vomit your guts out instantly." Dr. Turin is an odor theorist.

cooked kidneys ugh

This Flower..... The Amorphophallus titanum, also known as the Titan Arum, is native to the equatorial rainforests of Sumatra, in Indonesia. It has been called the "largest flower in the world," with a bloom that can grow to over 8 feet in height, opening as wide as 4 feet in diameter. When in flower, it exudes a foul stench that smells like rotting meat.

Isonitriles Here is a clip from: http://www.world-science.net/othernews/060925_odors.htm Chemistry defeats the “Godzilla of odors” Sept. 25, 2006 Courtesy American Chemical Society and World Science staff In the realm of stinkers, a group of che­m­i­cals called isoni­triles may have no ri­val. Odor the­o­rist Lu­ca Tu­rin once termed iso­ni­triles “the Godzilla of smells” and the world’s worst odor. “The smell is in­de­scrib­a­ble. Not that it is so un­god­ly be­yond all ex­pec­t­ed lev­els of stink, which it ap­proach­es, but more it is like noth­ing you have ev­er ex­pe­ri­enced and your brain while cer­tain­ly re­pulsed to the ex­tream [sic], strug­gles to de­fine it. A dou­ble as­sault at your sen­ses. It has a per­sist­ent lin­ger on ev­ery­thing that comes near it in­clud­ing your [brain cells]. Much like a hor­ri­ble pic­ture... that can’t be purged from your mind. Even af­ter the scene changes you are left with a ghost dif­fi­cult to erad­i­cate...” “I have no good de­scrip­tion for it. Be­sides me, the two oth­er peo­ple near­by, who were brave enough to try it... had the same thing to say, 'I can't de­s­cribe it but it sure is hor­ri­ble'. There does not seem to be a way to smell it on­ly slight­ly... I mean if you get en­ough of it to reg­is­ter, it’s en­ough to sat­u­rate your sens­es... In­i­tial ex­po­sure goes like this…You know it’s go­ing to smell so you try very hard to not get a big whiff by wav­ing the stop­per... get­ting brav­er every at­tempt un­til.. Bin­go! It’s ac­tu­al­ly fun­ny to watch some­one’s re­ac­tion when it fi­nal­ly sets in. It’s a very strange face. ... It took an hour and a half gal­lon of IPA99%​/​HCL32% to clean that smell out of my hood. If you do it, plan to spend some time de-stink­ing... Ti­ny whiffs of rem­nant traces on the hood tended [to] re­mind my nose of the full scale event and I dreaded it. — Descriptions of an isonitrile written by readers of science-madness.com, online forum for amateur chemists “You can’t be­lieve how aw­ful they smell,” Tu­rin said. “They make you vom­it your guts out in­stant­ly.” They’ve even been patented for use as non­le­thal weapons.

Esophageal reflux halitosis the day after someone has eaten salmon. It's sort of like garlic, but much worse, and vigorously repulsive

I hope I haven't smelled it yet....

There is little question in my mind that the worst and most persistant odor possible occurred when the wife of the Chairman of the Board where I worked had a long couse of chemotherapy. This rendered most of her salivary glands malfunctioning for a period of about a month.Her breath was intolerbale for human inhalation, even at 100 feet I can only attempt to describe it. Others described it as a combination of female ginko seeds, vomit, infant feses and sulfur combined, and on fire. Her perotid glands excreted a heinous fluid, which since located behind the jaw on either side, placing it in perfect position for dispersion by both breathing and talking. Subsequent enzyme treament halted the stench until the glands healed and began funtioning normally.

A guy by the name of T.J. Thompson of Indiana.

The base chemical called ammonia.

Michael Moore's armpits.

I don't know what it is, but The New Jersey Turnpike has it.

rotting fish guts

Rotting meat

A skunk... or a dead animal! ughh