I can't really relate favouritism to my own upbringing as I was brought up separately from my siblings, though perhaps that was a sort of favouritism in itself as, being an 'only' child, I had a more indulgent childhood than they did.
But although we never favoured any of our three sons my mother most certainly favoured our eldest from the moment he was born. She thought we should be content with just one child so when our 2nd son arrived two years later she was very critical of him, saying he was too chubby and why was he blond and he was nothing like his lovely brother! You can imagine her reaction when we had a third son three years later. Shock and horror, what were we thinking of! Whenever she visited she made a huge fuss of our eldest, bringing him gifts and giving him money and openly saying he was her favourite, but treating his brothers as an afterthought. Our youngest didn't seem to notice this, but our middle son did. He was polite and friendly to my mother but avoided her by going to his room. He asked why Nanny didn't like him. He became rather introvert, which sadly stayed with him till recent years. I didn't try to compensate for this unfairness in material ways, but I was always more protective of him. I, of course, challenged my mother about her behaviour and asked her to treat all the boys the same. Her opinion was the eldest child should be the most important in the family as they would be the one to care for you when you got old. How selfish is that! And I knew that was how she saw my role in her life, it didn't happen.
My mother's attitude was probably the result of her own upbringing. She was the eldest of two sisters, a bit plain, shortsighted and not very academic or creative. Her sister was the opposite and their mother's favourite. And, yes, my mother lived with and cared for her parents until they died.
We don't have any favourites amongst our grandchildren (natural or stepGC) and I hope we treat them all the same and love them equally.