Insect. In fact I think the actual answer was ants.
Don't ask me where this theory comes from as I've no doubt read it somewhere but basically I heard that insects would take over the world and that ants would be the dominant insect as they are the only animal other than humans to wage war and take slaves.
On another level you could say aquatic as obviously water takes up way more of our planet than the earth.
Basically, as long as we don't all go blind and them triffords don't stard marching I'd go with the bugs.