Determining the "worst-smelling" organ is subjective and depends on several factors, including the presence of infection or disease. However, based on available information, the gastrointestinal tract is a strong contender. While internal organs generally don't have a noticeable odor in healthy individuals, as noted in a Quora discussion about hypothetical organ smells (https://www.quora.com/If-hypothetically-we-could-smell-the-interior-of-our-body-what-would-the-different-organs-etc-smell-like), the gastrointestinal tract, extending from the mouth to the anus, frequently exhibits unpleasant odors due to the processes of digestion and bacterial activity.
While body odor is often associated with the armpits (https://my.clevelandclinic.org/health/symptoms/17865-body-odor), this is primarily due to a combination of sweat and bacteria on the skin's surface rather than the odor of an internal organ itself. The smell of the armpit is also a surface area smell, and is affected by genetics, diet, and hygiene practices. A related article points out that the smell is mostly present in the armpit area and relates that to human bipedalism. This demonstrates that the smell is the result of surface activity, not the internal organs themselves.
Other sources mention bad breath (halitosis) as a common and often unpleasant smell, originating from bacteria on the teeth and tongue (https://www.mayoclinic.org/diseases-conditions/bad-breath/symptoms-causes/syc-20350922). Again, this is a result of surface bacteria and not the smell of an internal organ. This is located within the gastrointestinal system, supporting the gastrointestinal tract as the most likely candidate.
It's important to note that many sources focus on body odor and breath odor, which are primarily related to surface bacteria and external factors, rather than the intrinsic smell of internal organs. In healthy individuals, the internal organs generally do not emit strong or easily detectable odors.
