That's easy. It's whatever opening you are most familiar with.
What's the best opening?
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 October 2019 issue of Chess lists the top twenty openings compiled from a list of 2746 August 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: 148 Retis, 131 declined Queen's Gambits, 126 Caro-Kanns, 124 King's Indians, 107 Nimzo-Indians, 105 Slavs, 86 Najdorf Sicilians, 80 1...e5 Englishes, 68 Classical Gruenfelds, 61 1...c5 Englishes, 54 1...Nf6 Englishes, 53 Queen's Indians, 50 1...e6 Englishes, 50 Giuoco Pianos, 48 Kan Sicilians, and 43 Berlin Ruy Lopezes.
"... 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)

That's a great answer that kindaspongey gave, but if you're interested in reading more, this question has come up before in the forums. I did a search for you...
https://www.chess.com/forum/search?keyword=best+opening%3F
My opening has always to compliment the lady on her nice shoes ( unless she is not wearing any). In which case you can compliment her toes.

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 October 2019 issue of Chess lists the top twenty openings compiled from a list of 2746 August 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: 148 Retis, 131 declined Queen's Gambits, 126 Caro-Kanns, 124 King's Indians, 107 Nimzo-Indians, 105 Slavs, 86 Najdorf Sicilians, 80 1...e5 Englishes, 68 Classical Gruenfelds, 61 1...c5 Englishes, 54 1...Nf6 Englishes, 53 Queen's Indians, 50 1...e6 Englishes, 50 Giuoco Pianos, 48 Kan Sicilians, and 43 Berlin Ruy Lopezes.
"... 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)
I was at a lecture and simul by Sam Shankland last night, and the question was brought up about playing one opening versus many and he specifically said the polar opposite of Jeremy Silman. A chess player needs to be well rounded, and not only should you not play the same opening over and over again, but you should be playing a line that emphasizes your weakness and should be working specifically on your weaknesses.
"... 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
... what's the best opening???
"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. ..." - IM Jeremy Silman (January 28, 2016)
https://www.chess.com/article/view/opening-questions-and-a-dream-mate
I repeat, what's the best opening???