Wow, what a question! God knows I've spent a lifetime looking for it.
It's almost easier to ask what doesn't work, and look for the white space in between to figure out what does. I mentioned earlier I don't like RPG's that are too tied to the RPG creator's world (like DragonAge or The Strange). I don't like RPG's that have 37 pages of gun porn (like Edge of Empire). I don't like games that take themselves too seriously as some transcendental experience, but also don't like games that are too break-the-fourth-wall goofy. I don't want six page character sheets, but I want an RPG that gives me enough rules-wise to be able to distinguish one character for another in terms of game mechanics. While I don't mind the odd bit of moral ambiguity, I want an RPG that gives the players the opportunity to choose to have their PC's act like heroes, because in the end I'm the grown-up version of the kid who liked to read comic books and mythology and Hardy Boys and the Three Investigators and who feels that there is enough brutality and ugliness in the world and I don't need to bring it into my imagination. I believe, perhaps optimistically, in the fundamentally virtuous nature of humanity, and I want what I create to reflect that. You may disagree, and I'm sure you can find your RPG to reflect that.
There are games I like, even love. But the perfect game, the game that I would want to bring with me on a desert island if it was the only came I could have? I haven't found that one yet. Might not even exist. We'll see.