It seems ironic that a simple, moulded, doughnut shaped part made of a single material could warrant so much attention.
The selection of a specific O Ring is based on a variety of factors that include:
Fluid media resistance.
Temperature range.
Hardness.
Durability.
Gland size.
Static or Dynamic application.
and much more. . . . .
There have been some significant advances in materials over the years, but often the best solution is the simplest.