Exposure to sunlight and/or atmosphere ozone is the worst in general.
After that it'd be the remaining fuel in the fueling system.
Some might say the above two should be reversed in terms of what is worst, and I couldn't disagree.
The engine seals are gasket type except for the transmission output shaft, which is extremely well shielded from any light.
The entire fuel system's elastomer sealing components, or acting components such a fuel pressure regulator's diaphragm, will be by far the most at risk of degradation only remedied by replacement.