If you need a time frame, let's say 25 years.

EDIT: my question is plural, but of course it's fine to give a single species if you know of one

Homo Sapians

Realistically : bees and other polinetor

Bees. And then finally stupid humans

Definitely the highest priority for the environment’s well-being.

I see this thread is already over. Moving on...

Better move fast, there’s not much time left.

We don't know enough, the whole biology on earth doesn't know enough to say for sure:

"We could definitely do without this species, or that"

Implicit in your question is the assumption that environmental protections are effective, or that they’re even based on any objective importance criteria, apart from say economic importance.

Plankton

Primary producers (photo-synthesizers and chemo synthesizers). All other life gets energy from them.

Edited in response to OP's edit.

It depends on how you define "environmental protections". If you're talking about actual laws (in the US, because that's what I have real world experience with) that establish management of hazardous waste (RCRA); force polluters to either remediate or pay to have their own pollution remediated (CERCLA); mandate clean air (CAA) and clean water (CWA); and ban manufacture, import, and production of toxic substances (TSCA), then those protections going away would be devastating. People as a whole forget how awful environmental conditions were 50 years ago before agencies were established and laws enacted. Obviously, these things are not universal, but the laws are sound and effective. The recent gutting of enforcement of those laws and prevention of implementing new protections will have lasting effects for generations.

The most important single species? I don't think there is one. Pretty much any one species disappearing would have its niche filled by another. There are a number of fragile ecosystems where that wouldn't happen and there would be a local collapse, but those would not have large-scale impacts.

If all environmental protections were removed, we'd see broad destruction of the environment across many species. Think broad deforestation, mining, waterway contamination, air pollution. It wouldn't be species, it'd be whole ecosystems killed.

I had plural in mind, but single is good too if you have one.

Most important to humans?

Humans.

Really if all protections were removed, the soil, water, and air would be so badly polluted with heavy metals, radiation, etc., from large corporations dumping that we wouldn't have the ability to grow enough food, probably within a generation or so. Long before climate change dealt its deadly blow. And the wealthy who survive that won't have enough servants to keep them alive for too long.

Most important to the Earth?

That would be more interesting and it would be really difficult to pick just one. Probably something like Prochlorococcus which is one of a few ocean plankton that produce the most oxygen on Earth. That will die off much sooner than Humans from climate change as the ocean temperatures and acidity are rising more quickly than the air.

As a side note, one thing I think a lot of people miss from the movie Soylent Green,...

Algae. It makes more than half of our breathable oxygen, some studies say up to 70ish%.

The question is about species going extinct. Despite all our laws, environmental devastation is still driving countless species to extinction, it’s just happening in other countries.

Mosquitos.

Other pollinators include flies, moths, butterflies, birds, bats, and rodents to name a few.