Ruy Lopez, Scotch Gambit, or mainline Scotch Game (3...dxe4 4.Nxd4) as White for the developing low-
Ruy Lopez is the most instructive long term, Scotch is very concrete, Scotch gambit would be a good choice for a developing player.
I would suggest one of the first two. When you reach higher levels, you could legitimately continue to use the Ruy Lopez or Scotch.
The Scotch Gambit is nothing for White if Black has any clue what he's doing, and in most cases, when you start facing players above 2000 over the board, they kinda know what they are doing! The problems a 2000 has that a 2700 doesn't isn't knowing opening theory. It's stuff that's way deeper than that!
Personally, my results improved tremendously against 1...e5 once I switched from Ruy/Italian set-ups and adopted the Scotch.
Longer answer, everyone's told me that the Ruy is the best opening, it's an opening for life, it will teach you about chess, make you a better player, cure your baldness, etc etc. I could never fully get it to work. I was inconsistent: good wins followed by terrible loses. In many positions I didn't know what to do, even though later I'd discover that 85 different GMs had all played the exact same position. Finally, the positions weren't fun or intuitive to me.
When I started playing the Scotch, a new world opened up. I consistently reached open positions that I enjoyed. I almost always knew what to do in the resulting middlegames. My consistency shot way up, and 1...e5 has become my favourite response to my 1.e4, where previously it was the worst. As a bonus, it seems most people are less familiar with the Scotch than the Ruy, and silly mistakes are common, mistakes that these players would never make in a traditional Italian that they've played hundreds of times.
Here is my approach to the Scotch. I don't know much more than this, in all honesty. Unlike what Skye has mentioned above, I have not found myself drowning in theory.
You can't go wrong with either the Scotch or the Ruy, imo, so see which one 'speaks' to you more. Maybe play 10 games with each, see how the resulting positions feel and then make a long-term choice.
scotch has a lot of great opportunties if they take the bait. i have run into trouble in end game though when i find myself down 3 or 4 pawns and my opponent starts pushing them all down. have to get the xmate before it gets to that point.
Excellent ... Each choice got an affirmative and a detraction by somebody!
Thanks for the answers everybody, especially SmithyQ and DierdreSkye for the detailed, nuanced, and personal insights ... I have some soul-searching to figure out my next move in my next OTB game where my opponent answers 1.e4 e5 2.Nf3 with 2...Nc6. But I will try not to worry too much about it, play some games with each, and go primarily for learning simpler fundamental concepts (like counting and not losing track of hanging pieces).
Thanks for the positive words, SeniorPatzer and DierdreSkye ... Got my queen trapped in one game, dropped a pawn in another game (but somehow managed to get a surprising draw offer in a lost endgame), and hung my knight in the endgame of my last game of the tournament. I don't know at what level openings start to factor in these tournaments, but it wasn't at my level.
As you play one serious tournament each year why not learn Ruy Lopez in the year before your next tournament and the Scotch next year. Then you might feel clearer about which is best for you. Also, it will keep your opponents guessing, they will not be able to label you as the Spanish guy.
I just missed a one move rook sack and a two move bishop sack in a winning game of 15|10 that I eventually lost in a miserable fashion - his white bishop simultaneously attacked my rooks on a white diagonal... obvious yes... I knew to avoid this.. but did I? No...
So, yes, definitely concentrate on getting that simple stuff right! The rook was far away along a long diagonal, and I wasn't thinking about that possibility as I was concentrating on attacking the King. The Bishop sack involved a bayonet attack from one of the King's house pawns, and I don't like moving them, but I should have moved that one! All this after an error free Sicilian opening that I spent some time, probably too much time, studying. So frustrating. How do you get better at "considering all tactical possibilities"? Keep on playing games of 15 |10 and focus on the tactical blunders that computer analysis reveals?
Anyway, I think is one example of why the Masters keep on saying beginners/intermediates shouldn't think so much about the opening, concentrate on tactics. If you spend a year learning the Ruy and then miss a rook sack how upset will you feel. So maybe spend a month studying the Ruy and eleven months with tactics trainer and playing 15 | 10 with computer analysis. It's far less frustrating to lose to someone who knows the opening better than to lose through a daft blunder.
Play the opening you like best. It does not matter too much which opening as long as it is a sound opening.
Scotch is great. And often players above 1800 do not know how to defend this opening. Even masters, playing Black, can and do go wrong sometimes against the Scotch.
So if you decide to play the Scotch just througly study that opening and go into data bases to see how strong players have played that opening.
Here are reasons I have heard for each, and I am interested not only in whether you affirm these reasons, but more especially any unique reasons why you have an opinion of one over the other ...
Ruy Lopez: mother of all 1.e4 e5 opens, tactically and strategically rich (is that an argument for or against a low-level player playing it?), lifelong repertoire
Scotch Game (3...dxe4 4.Nxd4): a low-level player needs a sound repertoire that is simpler and less theory than the Ruy (I don’t care about the amount of theory since I won’t be studying it other than maybe books like “First Steps”, “Starting Out...”, or maybe just FCO).
Scotch Gambit: a low-level player needs to learn attacking and initiative.
For background, I just played in my first OTB tourney in nearly a year (6th in the last 5 years), scoring 2.5/5, and played my first “classical“ openings OTB in 5 years (I had previously played the London as White, Scandinavian against 1.e4 as Black, and never once got a 1.d4 as Black in the few tourneys I did play).