Objective right and wrong can not be just what is best for me. That's not morality. You are right back to the biggest and meanest person being able to say what is right for all and what is wrong with that mindset.
Real morality is actually just the opposite, what is best for everyone else?
Sorry, that cannot work. You might have gotten some hints in the other posts.
You assume that "the best" for someone is a kind of zero-sum-game. What is "best for me" must necessarily mean that it is "worse for someone else".
Ok. Let's say this is correct, for the sake of the argument. But that would mean that what you say is "real morality" means: "what is best for everybody else" is "worse for yourself". You don't get any gain from "being moral". The reasoning you used to reject the "what is best for me" morality prohibits that.
But no moral system is that simplistic, nor is there an absolute limit of "good" that can exist.
You claim that it would be the "biggest and meanest" person dictating morality for all. That has a massive problem I will get to later, but for now: why would that have to be the case? Could it not be the smartest and most caring person dictating morality for everyone? Just consider: this is basically what
your theistic model of morality is claiming. This person is "God".
So even within a "what is best for me" system of morality, this doesn't have to mean robbing everyone who is not me... it could also mean ensuring that everyone gets "what is best for them". And that even might include (we know it does) sharing and cooperation.
You have only that idea of such a system as "me against the rest of the world". This is a very short-sighted view.
As the Bible says, 32 “If you love those who love you, what credit is that to you? Even sinners love those who love them. 33 And if you do good to those who are good to you, what credit is that to you? Even sinners do that. 34 And if you lend to those from whom you expect repayment, what credit is that to you? Even sinners lend to sinners, expecting to be repaid in full. 35 But love your enemies, do good to them, and lend to them without expecting to get anything back.
And that leads us the the problem I have mentioned above. The system I have talked about above is not "perfect". It can work - it does work, we know it does - but it can also be sabotaged and manipulated.
But so can this system here. The system you call the "real morality". They system you say is meant to ensure "what is best for everyone else".
The problem: it doesn't. There is nothing in this system to prevent the "biggest and meanest person" to game it, exploit it... and get to the very system you reject. Even easier than a system build on cooperation.
Here is it, written in plain words: "lend to your enemies without expecting to get anything back." That is, in nice words, saying "Allow yourself to be robbed. If you don't do that, you are not better than a sinner... a sinner yourself. Immoral."
Try it. Right now, I need a few thousand dollars. Say, 8000. I am nicely asking to "lend" them to me. I, the immoral atheist, promise to pay you back. Pinky swear!
Let's see... you wouldn't send me money, right? You're not willing to follow this "real morality" of yours... because you know it would lead not to "what is best for everyone else", but "what is really bad for yourself".