What are the best openings that should be taught to beginners in order for them to improve and become strong?

Here "best" doesn't mean best to maximize their winning probability in the immediate short-term, but best for them to improve and eventually become strong players. Only the long-term improvement matters. The short-term improvement is irrelevant. I have heard that adopting a system (London, Colle, KIA, etc.) would be good in the short-term but bad in the long-term, because it would limit the range of positions that they would get exposed to.

My goal is not for them to get an advantage out of the opening, but to give them openings that will often lead them to some types of positions which will be particularly instructive to them and will make them improve faster.

The players under consideration here, the so-called "beginners", are players who are below 1100 Elo, players who do not yet know what openings to play, players who do not know anything about openings.

Should each individual beginner play openings that fit his own particular style or preferences? Or is there some particular type of openings that is best to play for all beginners regardless of their individual style and preferences?

What kind of openings should we teach them?
- mainlines or sidelines?
- classical or hypermodern?
- open or closed?
- tactical or positional?
- sharp or quiet?

Most people say: mainlines, classical, open, tactical, sharp.

Most people say: 1.e4 e5 (the Open Game, aka the Double King's Pawn) with both colors. Okay, but what should we teach them to play on move 2 and move 3? And what should we teach them to play as Black against 1.d4, 1.Nf3 and 1.c4?