I have done a bit of study on chess openings and especially which openings are best especially for beginners. So lets look at our aims of the openings and why they are played.
Chess is a game of two armies perfectly balanced at the start, each side has the pieces hidden behind a wall of pawns. So simply White breaks that balance and Black has nothing to 'react to' since White is yet to play a move.
Thus the first moves are played simply to find out how the enemy is going to play and to develop an advantage to be exploited. After these first 3-4 moves now is the time to decide if you are going to seek more board space, gain material (captures) or need to defend from threats or begin an attack.
But most importantly you need to DECIDE on a PLAN of action early on and how you are going to continue your next moves with your goals outlined.
Thus White sets the stage with the first move (the inititive) and Black 'reacts' to these moves until they can gain the inititive and decide how to implement their plan; this is why White wins 55% of games and Black only wins 45% of the time.
Thus it is fairly easy for White to decide on a first move but much harder for Black since they must wait to see how White will upset the balance. So it would be fabulous if Black had a perfect opening and could ignore White for a few moves but such an opening does not exist, but some reliable openings can come close.
A mixture of the Caro-Kann/Slav defenses are solid and reliable and proven as workable at all levels of chess. So if these Black openings are so good why are they not universally accepted? Simply they are less exciting or boring compared to the popular Black openings; people play chess for fun mainly so why play a boring opening lol.
Simply White can live with one main opening for much of their career but Black cannot use just one opening in return to anything, but a mix of the Caro-Kann and Slav would serve you forever.
But another way is to play the Nimzowitsch Defense or Modern Defense. These two openings are less reliable but wonderful for another reason, they win games over time by three methods. 1) They surprise the enemy so your understanding of your opening will be better than Whites ability to adapt, 2) They can follow independent lines unless you wish to bring the game back to a more conventional opening after seeing how White is going to try and direct the game. But the most important is the third reason 3) they force you the user to learn more about chess overall by simply ignoring most of the huge amount of opening theory that is available today.
If you play the Modern Defense or Nimzowitsch you develop your natural chess playing ability and then use this ability to win games, people will tell you these openings are not solid or reliable but many top players have achived top results with these two killer Black weapons.
So my suggestion is learn more about chess overall by avoiding most of the main stream openings and learn to play chess well yourself, focus on avoiding errors and force your opponent to play for themselves instead of stupid memorised best moves.
Play the Caro Kann /Slav mix and look int othe Nimzowitsch or Modern Defense and turn them into your killer Black weapons, all four are certainly strong enough and reliable enough for your entire chess career. Others who say they are not are simply misguided...