Kyr, we technically are capable of going to another star system now, its just so stupidly inefficient that it would be a matter of generational ships. Most of the feasible transport tech we can see in the future doesn't improve upon that situation by much.
Because of all that, I'm assuming that first contact with another race would be a handful of techy guys in a metal pod that cost a notable portion of their world's GDPG
But if we are talking about someone so far advanced that traversing the distance is easy enough to make war both doable and worthwhile… Nanites or bioagent, designed to wipe out humankind while leaving everything else intact. Sweep the planet clean of inhabitants for a colony.
And that's if they want Earth's ecology intact. If it is incompatible with their needs, they could just start terraforming and let us die off as the temperature changes significantly, or atmosphere gas mixture shifts. Edited: daishain on 14 Hours Ago