This is an opinion based question, and the answer is going to be based on an individual's own reasoning and support.
Personally, I believe that the most important factor in any given ecosystem is sunlight. Sunlight and its related thermal energy drives weather patterns, temperatures, winds, evaporation amounts, etc. Additionally, sunlight is the initial energy source for all food chains and food webs. Very few producers exist that don't depend on sunlight and performing photosynthesis. Without the sun, there are not producers, and consequently, the following consumers would die.
Environmental pollution is affecting the sunlight that reaches an ecosystem. Environmental pollution takes many forms. One such form is air pollution like smog. The particulates that are in the air pollution reduce the amount of sunlight that reaches Earth's surface. Less sunlight means less photosynthesis, and that reduction in energy will be felt throughout the entire food web as only about 10% of the total energy that is present at a given trophic level is passed on to the next trophic level.
Another environmental factor to consider as crucial to an ecosystem is water. Like sunlight, water is still an abiotic factor. Like sunlight, water is absolutely essential for photosynthetic organisms because photosynthesis uses sunlight energy to convert water and carbon dioxide into oxygen and glucose. Environmental pollution can harm water sources within an ecosystem, and those tainted water sources can damage the producers which will then ripple through the rest of the food web.