It's difficult to definitively say what the single strongest smell by intensity is, as smell perception is subjective and varies greatly between individuals. Factors influencing smell intensity include:

* Concentration of the odorant: Higher concentrations generally lead to stronger smells.

* Individual sensitivity: Some people are more sensitive to certain smells than others.

* Adaptation: Repeated exposure to a smell can decrease our perception of its intensity.

However, some substances are generally considered to have very strong and pungent odors, such as:

* Hydrogen sulfide (rotten egg smell): This gas is highly noticeable even at low concentrations and can be dangerous in large amounts.

* Mercaptans: A class of sulfur-containing compounds often used as odorants for natural gas, they have very strong and unpleasant smells.

* Skunk spray: This noxious secretion contains sulfur-containing compounds that are incredibly potent and difficult to remove.

* Ammonia: A strong, pungent odor often associated with cleaning products.

* Formaldehyde: A colorless gas with a sharp, suffocating odor used in embalming and some industrial processes.

Ultimately, the "strongest smell" is a matter of individual perception and the context in which the smell is experienced.