Is 1.e4 the best first move for White from a theoretical point of view? Assuming that the principles of classical chess in the opening are still valid (which thing I very much believe), may we say that "from a theoretical point of view" 1.e4 is the best first move for White? (Occupying the center, freeing two pieces). No other first move can do that.
According to some GMs, d4 is the best first move.
Alekhine, commenting on a Slav in his greatest games book, claims it gives White more chances of gaining an advantage.  After taking the c pawn, Alekhine further comments that White now obtains an appreciable advantage. (Quoted by Hans Berliner.)
Hans Berliner, in his book The System--A World Champion's Approach to Chess, claims, and Fischer agrees, that the advantage of the first move is the ability to attack the opponent's pawn center first.  Since both agree that the King's Gambit is dubious, that leaves the Queen's Gambit as the only sound opening.

Many GMs have said d4 is better:  although they claim it is due to needing a deeper understanding of chess.  Anna Zatonskih states it better that you can make some minor inaccuracies and still have a good game.  In e4 games, one mistake could cost you the game.