Best chess opening

What is the best chess opening?

No way

Define "best". Most sound? Most aggressive? Most positional?

Most aggressive 

If you're going for maximal aggression, it's hard to beat the Double Muzio Gambit, which involves sacrifices of both a knight and a bishop in exchange for a whole lot of attacking opportunities.

Is it the best opening? Probably not, and it's not super popular these days. On the other hand, it can be very fun to play.

Yes

On Grandmaster level it is probably the Ruy Lopez.
Below Grandmaster level it does not matter: you do not win or lose because of the opening, but because of blunders and tactics.

Ok 

The Nakhmanson Gambit is a surprising, aggressive knight sacrifice in the Italian Game. This is good to play at under 2000 level for a good attack.

Bird's opening is strategy and aggressive opening.

the most aggressive opening is the kings gambit (it is also very risky. if you like risks you should consider it. I do not do it, but I do other gambits

I kind of prefer the Vienna Gambit, but it's up for the debates. There is no best opening, but there are some bad openings which everyone should avoid using