what is the best opening?

No such thing exist

According to Stockfish, the following first moves are the best: 1. e4, 1. d4, 1. c4, 1. Nf3 and 1. g3.

After the first move, your opponent gets a choice. If you get black, you only can react to whatever first move white picked.

Typically people take a choice and stick with it. For example I always play 1. d4 as white and as black I play 1. e4 c6.

After that a beginner, thats players until about rating 1000, should follow the general opening principles:

- Develop all pieces into active positions.

- Castle to a safe side.

- Open the position, because beginners mostly focus on tactics. Also open lines are ideal for your rooks.

Really strong players of course know a massive amount of opening theory.

E4 best by test.

It's a well kept secret. Top GMs play other openings only to fool you.