If you're interested in improving a little bit more (regardless of what your current level is) that's when, IMO, you can choose things like the Scandanavian which sets the tone early and has minimal theory.
For longer term improvement an opening like main line Ruy Lopez is great. The middlegames are very rich, and there are many different ideas among the many different variations.
The queen's gambit declined, aiming for e.g. a tartakower is great. If you learn how to handle those structures it makes learning other 1.d4 defenses easier like the Nimzo... but starting with the solid and logical QGD has the benefits that it's... well... solid and logical
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You mention lines and ideas seemingly as if they're the same, but remember the whole point of proper opening study is having an eye on the middlegame ideas. Go to chessgames.com and load up 50 games from the Botvinnik - Fischer time period (all from the opening you want to study), and then 50 from more modern period. Play over them really quickly and note things like which side of the board did each player seek play on (kingside, center, queenside). Note the common pawn breaks. Note common piece arrangements, endgames, tactics. This should only take a few minutes per game, and you can easily do 100 games in a week.
At first this might be a little slow going. Maybe in the first 10 games they're all different variations and there's nothing too similar... but the more games you see, suddenly you'll see common ideas and patterns.
After that it's fine (and useful) to memorize lines... and this goes for any opening not just Ruy and QGD.
Anyway, good luck.
What is most beneficial for a player wanting to improve (past the 1800 threshold, for example--I realize that this question is highly elo-specific), understanding ideas behind many openings or knowing in-depth lines in a few openings? Also, before anyone points this out, I realize that improvement revolves around many elements in chess, so please limit discussion to opening theory.
Certain lines in chess can be avoided entirely if someone wants (for example, I never reach the Ruy Lopez 1.e4 e5 2.Nf3 Nc6 3.Bb5 because I do not play 1...e5 as black), so is there ever a reason to learn them? I believe that understanding the ideas in some of these openings that I never play is beneficial because other openings may transpose or utilize similar ideas, but at the same time I can avoid them entirely if I want.
I am interested in hearing people's thoughts on this somewhat philosophical question. Insight from higher rated/titled players on what has worked for them would be greatly appreciated.