Mosquitoes, fleas, ticks, etc.
Here someone will say "but that will damage the fragile web of life and kill mother gaia" or some other drivel. The biosphere is quite stable and can survive the loss of all these species without crashing the food web.
Look at a remote tropical island that doesn't have any of these species. They exist. And guess what... they're fine. Will something that eats these things likely have a harder time finding food? Sure. But if the species isn't already on the brink of extinction then it will adapt. If it was already on that brink then chances are it was doomed in any case. Adaptive species don't get into positions like that.
And beyond that, species go extinct all the time. Always have. New species fill their niches or existing species simply expand to fill vacant niches which tends to cause them to splinter and create new species.
Here again, someone will say "but the rate of extinction has increased!" Yes it has. The biggest reason is human transport and trade. You let loose a rat from the mainland onto some little island and it is probably going to go sickhouse on the local species that likely haven't had to work as hard to survive on their little island. And yes, that rat or other relevant more vital species is likely to eradicate or out compete its rivals. Evolution at work.
Beyond that, we are also destroying habitats. And that is sad... We should try to limit that sort of damage when and where possible. However, the mosquito can go fuck itself sideways with a rusty chainsaw. I have literally zero sympathy for that species. And I am quite comfortable geo engineering the world to the extent that nightmare species like that simply don't exist. Mother nature has come up with some very impressive things over the eons. But she has also birthed some monsters. And I am quite comfortable aborting those little experiments.
Here again, someone will say "but humans are the biggest monsters"... then kill yourself. Shut up and kill yourself. I have zero patience for that drivel.
Kill all the mosquitoes.