Which opening is the best?

I think e4 is the best move to start with but what it the best opening?

There is no 'best' opening,

It doesn't take a genius to figure this out: if a 'best' opening existed, why would any of the World-class Grandmasters play anything else? Wouldn't all of the GMs be playing the same 'best' opening all the time?

The best opening is the opening you understand best. The opening you have the most experience playing.

(as long as at least 2 different GMs were willing to use it at least 2 different times each in an important classical game  in other words the queen's gambit is definitely good, and the Latvian is definitely bad)

Openings are not a one sided affair. You can only play an opening with your opponent's cooperation.

For example if I want to play the king' gambit, but my opponent plays 1...c5, then I can't play the king's gambit.

An exception to this are system like the London, which can be played against almost anything your opponent does. Systems are equal at best... obviously there's no such thing as 1 way to open a game that consistently leads to an advantage, otherwise chess wouldn't be very interesting.

A "gambit" involves giving up (or at least, offering) some material such as a Pawn, in return for some compensation such as better development or better center control.

Example: King's Gambit 1. e4 e5 2. f4

A "system" is a sequence of opening moves that you can play almost irrespective of the opponent's opening choices.

Example: King's Indian Attack 1. Nf3 followed by 2. g3 and 3. Bg2, with 0-0, d3 and Nbd2 to follow.

The word "game" is a rather neutral term, which makes no specific claim about the opening line.

Example: Queen's Pawn Game.

That was a game from a simultaneous display against an amateur (Fischer vs J Jones).

The opening that you know best is the best opening.  

Maybe they are students of Lasker Psychological Chess School? 

1. h4

Best opening? You can't ask that on a chess site - what are you trying to do, start a riot or something? xD

More seriously though, many chess beginners often inquire for the "best" move. Tricky to explain though, there is seldom a "best" move but often at least three "good moves"; strong players consider these options before electing their move, where the "options" are called "candidate moves". Many chess positions lack a single "best move", but may have 2 okay moves, perhaps a "better move", maybe a move that looks all right two moves deep only to be losing by a tactic etc. 

Openings are simply this on a larger scale. Some openings are simply "good" and are heavily studied - these are the "main lines". Of course, many sidelines are common/strong too but are perhaps a sideline due to some counter idea or undesirable variation. Experiment with many "reasonable" openings and see what you are most comfortable with. Out of the three stages of the game (opening, middlegame, endgame), the opening is the most forgiving. As long as you can basically develop your pieces out, and get a structure you like, the opening transitions to the "real" chess game. Surely some opening blunders cost the game, but these instances are rare compared to won endgames or a middlegame tactic deciding the outcome of the game. 

Test out some openings. In time you will see which openings give structures you like and type of play you favor. However, "study" is perhaps better spent on theoretically won/lost/drawn endgames and tactics (like chess.com "tactics trainer", at least on an in-depth scale until you reach a much higher rating; many chess players never get to this point. If you ask a 1600 chess player when they should deeply study openings: they often respond about 1800 rating. Ask an 1800 rated player the same question, and they often respond a 2000 rated player. It sounds silly but it is true.

The goal of the opening phase is to reach a middle-game position in which you feel comfortable.

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TINSTAABO

"There is no such thing as a 'best opening.' Each player should choose an opening that attracts him. Some players are looking for a gambit as White, others for Black gambits. Many players that are starting out (or have bad memories) want to avoid mainstream systems, others want dynamic openings, and others want calm positional pathways. It’s all about personal taste and personal need.

For example, if you feel you’re poor at tactics you can choose a quiet positional opening (trying to hide from your weakness and just play chess), or seek more dynamic openings that engender lots of tactics and sacrifices (this might lead to more losses but, over time, will improve your tactical skills and make you stronger)." - IM Jeremy Silman (January 28, 2016)

https://www.chess.com/article/view/opening-questions-and-a-dream-mate

The April 2018 issue of Chess lists the top twenty openings compiled from a list of 2706 February games where both players were rated over 2400 Elo. One can not take position on this list too seriously because it is greatly influenced by how the openings are grouped. For example, all the Retis are grouped together, while English is separated into 1...c5, 1...e5, etc. Nevertheless, for what it is worth, some of the list entries are: 214 Retis, 127 Caro-Kanns, 123 King's Indians, 84 declined Queen's Gambits, 74 Slavs, 71 Nimzo-Indians, 66 Najdorf Sicilians, 56 Taimanov Sicilians, 53 Queen's Indians, 52 1...Nf6 Englishes, 51 1...e6 Englishes, 51 1...c5 Englishes, 51 Classical Gruenfelds, 49 Giuoco Pianos, 47 Kan Sicilians, and 45 Tarrasch Frenches.

"... A typical way of choosing an opening repertoire is to copy the openings used by a player one admires. ... However, what is good at world-championship level is not always the best choice at lower levels of play, and it is often a good idea to choose a 'model' who is nearer your own playing strength. ..." - FM Steve Giddins (2008)