Our species? Obviously top. Other species? How important they are to our survival and (secondarily) our quality of life?

Note that from this point of view, you arrive at some measure of environmentalism. Solving climate change is much more important than random animal or plant species. But preservation of species is very important too, for reasons like ecosystem stability (we wouldn't want to stress nature too much or have a linchpin species go extinct, and make the planet less habitable in the process), drug research, food supply stability, and ultimately also aesthetic reasons.

So: climate change >> mass extinction >> occasional extinction. Microplastics I'd tentatively put between mass and occasional extinction, but proper placement depends on the outcome of further research.

I understand the implications of this question, but why do we prioritize self-preservation over all else?

Like, I get that's a biological instinct, but at some point it has to be not worth it to save ourselves. If we're making value judgments for everything else, it's simply wrong to ignore ourselves.