There are 5 candidates for best move
1.- e4
2.- d4
3.- c4
4.- g3
5.- Nf3
there are pros and cons to each of these moves and your personal interpretation of chess will have a lot to do with which one you consider best.
Many trainers and masters consider e4 to be better for beginners because of the more open positions arising from that opening (mostly). d4 makes it easier for white to take control of the centre but the positions arising are usually more complex (botvinnik, Mar de Plata, exchange grunfeld, Saemisch). The english can both lead d4 positions or independent branches and is fairly easy to learn but hard to master. Nf3 was considered best by argentine chess author Grau because it waits for black to reduce pawn flexibility before making a pawn move. I don't think g3 is as good as the other four moves but I don't have a concrete refutation.
Certain players like Hans Berliner or Sveshnikov have turned their carrier into a crusade in favor of their opening move and against others, while players like Smyslov or Larsen have played almost any move playable.
What is everyone's opinion on the best opening move for white?