In low elo you're right, though many things go into getting kills, like warding in their jungle or something like that. If you are playing with people you recognize to be not that great the most important thing to do is to get picks in the jungle or something. It can be difficult to get a team to capitalize on the brief advantage you have, but if you force a 5v4 or take a tower you should come out ahead.
But to answer the OP, mid or jungle. They have the most presence in the early game, and many games are pretty much decided withing ten minutes.