This isn't going to help you with your school report, I'm afraid, but I think the most important thing about the environment is that it's ALL important -- it's ALL interconnected, and of course our own lives and our society are part of the big web.
I think it was the conservationist John Muir who wrote that when we pick out any piece of Nature to look at, we find that "it is linked to the entire Universe."
However, the science of ecology, which is the science of how the different pieces of the environment fit together, does point to aspects of the environment that are more central to the whole interlocking web of living and non-living systems.
For example, animals - endangered and otherwise -- are certainly part of "the environment," but ecologists generally point out that the habitats of the animals, the places where they eat, mate, raise offspring and find shelter -- are essential to the survival of large animal populations.
So you might say that the survival of the Amazon rainforest, say, is more crucial than the survival of individual animal populations living in the rainforest. Similary, the survival of coral reefs -- one of the most diverse and productive ecosystems on earth, next to the rainforests -- is important to the survival of something like 1/4 of the fish that live in the oceans.
So maybe my "favorite" parts of the environment should be the tropical rainforests and the coral reefs, since they are so important to many other things.
Other favorite parts of the environment, for me, include the earth's atmosphere, which provides people and animals with oxygen to breathe, and provides plants with CO2 for the process of photosynthesis.
The earth's "water cycle" -- the environmental process that takes water from the oceans via evaporation, turns it into atmospheric water vapor, turns that into rain and snow, and turns the rain and snow into rivers that carry the water back to the oceans again -- is another big favorite.
The sun, meanwhile, even though it is 93 million miles from earth, is essential to the functioning of the water cycle, essential to the survival of the rainforests and the coral reefs, and the source of almost all of the energy consumed by living things on earth. So I'd say the sun is my most favorite part of the environment.