What can be the best move for the opening of GM
I would say the Grünfeld Defence, it starts with: 1. d4 Nf6 2. c4 g6 3. Nc3 d5 Black offers White the possibility of cxd5, when after ...Nxd5 White further gets the opportunity to kick the black knight around with e4, leading to an imposing central pawn duo for White.
ıt can be e4 d4 c4 b3 or Nf3
Look what a professional GM plays. That's the answer to your question and no secret.