The five best first moves are:

1. The one that causes your opponent to resign
2. The one that your opponent hasn't prepared for
3. The one that surprises your opponent the most
4. The one that you've been playing for decades
5. The one that you've written lots of books about

But possibly not the answer you were really looking for 

A quick DB report on GM games (I have about 110,000 in Megabase 2006) gives d4 at 56%, e4, Nf3 and c4 at 55% and g3 at 53%. But 86,000 of those games are d4 or e4.

However the highest scoring opening move is 1. a4! at 57%  . All of these 1. a4 games are worth analysing.

For example the first 5 best moves in master play are e4, d4, c4, nf3, and g3.

Then for e4 are c5, e5, e6, g6.  So and so on.

This can be answerd by begginers and master alike.

Thanks, lo0k

PS(If you are a master, please identify yourself saying: "That I am a chess master")