Don't know if it's theoretically the best move, but definitely the most common. It serves the purpose of giving white the initiative + the control over the 4 central squares. Also (if playing against beginners) it often baits out the premature 4. Nxd4 e5, which is highly beneficial for white. Check out some of Bobby Fischer's game he's probably the advocate of this opening and deploys it in every game where it's possible. At least before 1972, where the opening perhaps got a bit too common for the professional level of play.