Statistically speaking d4 is the best move, mainly because the Sicilian scores very well against e4. That of course doesn't mean you must play d4. If you like e4, play e4. Many top GM's do with great success.
if you're interested in properly learning chess, you should probably stick to the opening principles and play classical openings. Ruy Lopez, Queen's Gambit accepted/declined, Slav and the Sicilian.
The King's Gambit is fine if you're a top level player and want to throw your opponent off, but all offbeat and obscure openings aren't really beneficial, either for winning or for educational purposes.
The trouble with "best" is:
What do you mean with that? There is a "best" move theoretically, and there is a "best" move practically. The theoretical best moves might not be the most challanging for your opponent (which means it isnt difficult to find the best reply).
I believe that theoretically most opening moves lead to a draw, even s.th. awkward like 1.e4 f6(?) - it wont be enough of a weakness to win the game for white.
But practically I doubt 1.e4 f6 will give you great success.
TitanCG is certainly right: a move that is statistically very good, might have been good in the past, but meanwhile a refutation is found.
I am actually playing such a refuted opening as black, but I do so because statistically black scores better than white . The refutation is almost not known to people, but if I ever reach Master level, I might need to look for a different opening as black, especially if my games start to appear in databases. For my expert-level it is enough, though.
Not refuted against 1.e4 is:
1...g6 (The Modern) and 1.e4 c5 2.Nf3 a6! (O'Kelly), both openings are probably theoretically sound and practically excellent (better stats for black in most variations).
But that leads to s.th. I noticed often: the best moves seem to be the most flexible moves, that can still adjust to white's/black's play.
So no wonder that the very flexible 1.Nf3 or 1...g6 score so well. The disadvantage to this big flexibility is, that these practically best openings require considerable amount of theory to be learnt.