Curious on some replies here. I always hear having bees go extinct would be horrible for us. Curious if that’s the worse?
Humans.
That’s really the only right answer. If we’re looking at the impact of a single species, as opposed to a genus, family, or order (like most of the other answers are doing, e.g., “spiders”), humans are the only single species whose absence would cause vast changes in the biosphere. We have no other close taxonomic relatives that could step into our “niche” and continue doing what we’re doing. Losing one species of mosquito (instead of the whole genus) or one species of plankton (instead of the entire… god, what, order? Clade?) wouldn’t produce any significant effect by itself.
Are we being pedantic about the definition of species? Mosquitos from the Anopheles genus (and only those species) spread malaria. They’re humanity’s #1 killer.
Driving them and the other mosquito species that spread human disease (Aedes spread dengue, yellow fever, Zika, and chikungunya) should be seriously considered.
Bees, plankton, and our keystone microbiota species.
Many people here seem to suggest the likes of plankton, worms, trees, flies, bees or butterflies. But these are not just one species, they’re huge groups of organisms. It is interesting to think about extinction of whole groups, but it’s necessary to understand it’s on the same level as thinking about the extinction of all rodents or even mammals.
I’m not a biologist, but I feel like krill would be a strong contender, considering what percentage of ocean creatures rely on them for food.