The people who "don't see it" aren't looking. IME, the people who deny that parents have favorites were the golden child themselves and are likely to be guilty of the same behavior.
Agree that it's not always the oldest. My parents were both the youngest, and both the "golden children" in their families. Interestingly, my older brother was the favorite. My Mother even admitted it after I was grown, and apologized.
It hurt me for a long time, but I'm not bitter about it now. In the long run, it's not necessarily a bad thing to not be the favorite. I grew up to be a much more independent and successful adult. Almost to a fault, though. To this day, I have a hard time believing that I deserve help from others.