Depends on your angle: In term impact to the field of biological science, definitely Drosophila melanogaster. Lots of genetics, and now even brain research. I

Depends on your angle: In term impact to the field of biological science, definitely Drosophila melanogaster. Lots of genetics, and now even brain research. In term of most beneficial to humans: bees (direct economical benefits) and ants (largest environmental benefits, so indirect benefit to human) In term of biggest threat, the debate is open, and always changing: before mosquitoes (Anopheles and Aedes), houseflies can carry a lot of diseases of every kind (parasites, bacteria, viruses). At one point it was the flea (the Black Death). Grasshopper and crickets also caused a lot of people to die from hunger, and are still significant agricultural pests. Ants and termites (crops, food supplies, infrastructures, diseases...)