Best response to the Nimzo-Indian Defense (E50)

1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. e3 O-O 5. Nf3 · Black to move

What is the best response to the Nimzo-Indian Defense (1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. e3 O-O 5. Nf3)? Black to move. See the top replies ranked by opening theory and real-game results, starting with d5.

The main theoretical reply is d5, reaching the Nimzo-Indian Defense: Normal Variation, Ragozin Variation.