you can remove one insect from existence
you have to choose between of these
Neither, as they all keep the natural ecosystem in balance.
What I would get rid of however are any parasitic species that happen to have no natural enemy, like guinea worms. Fuck those bastards.
None of them.
The least retarded answer is mosquito though. Mosquitos are important, but spiders are important as fuck and more unique, and flies are one of the most important animals period.