Best response to the Réti Opening (A09)

1. Nf3 d5 2. c4 · Black to move

What is the best response to the Réti Opening (1. Nf3 d5 2. c4)? Black to move. See the top replies ranked by opening theory and real-game results, starting with b5.

The main theoretical reply is b5, reaching the Réti Opening: Zilbermints Gambit.