1. a3 is the best. ALSO, lets bring this forum back 
https://www.chess.com/forum/view/chess-com-community/hi-my-name-is-stockfish-ama-ask-me-anything-50020072?page=109
1.a4 is not the best
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... why is this forum still here
I think a3 is better. the reason is that it is a useful move in many variations. you let black have a move, but you just go copycat and use the a3 pawn to go for a minority attack
Not an easy tactic to say the least we have about two other openings in this regards I will prefer to go for a knight opening