Mosquitoes. They fill literally no ecological role in the biosphere - no animal would suffer were we to remove mosquitoes.
If I may, though, considering we are still finding new species of bugs that lives off of mosquitoes? It's far-fetched, I know, but not an impossibility. And if that species were to die out, there could be a chain reaction (although once again unlikely).
I just don't like to leave out even slight possibilities. ;)