I'm the eldest and I wasn't, and am still not the favoured child - that is definitely my sister.
Examples - I got a £21 clock radio for my 18th birthday, she got a £250 oboe. We had to share bath water, and she always got to go first, and have the hot, clean water, whilst I had the dirty, cooler water, and had to clean out the bath afterwards, and tidy up the bathroom after us both. When I asked mum if my sister and I could take turns at going first in the bath, she turned me down, point blank.
She never once came to any of the concerts I sang in at university, even though I was doing some pretty big solos. Dad came, and it would have been no effort for her to come, but she never did.
I have three children, and whilst we joke about who is the favourite, I love them all equally, and I am sure they know this. Dh and I are careful to ensure that they are all treated fairly, and that they understand our decisions. It was not always easy when they were younger - there were things the eldest was allowed to do, that the youngest wasn't - and we explained to ds3 that, when he was older, he'd be able to do the things that ds1 could do. At the time, he did think it was not fair, I suspect.