I definitely don't favour either of my children, but there's been times through both of their childhoods that I've had to focus a lot more on one than the other, which has caused resentment ( and an accusation of favouritism by my eldest). My boys are also totally polar opposites of each other, and there's a 5 year age gap - so both at very different points of life
My eldest is 15, he's strong willed , very clever , won't ever accept he's wrong, also very loyal, a wonderful friend. Aged 5-10 was a very turbulent time. He's my DS from a previous relationship, he doesn't have the best relationship with his Dad ( very basic EOW "Disney dad" ). He was hard to parent as a young child, very angry all the time
, used to fight us all violently. He's now turned into the loveliest teenager. Still strong willed, but secondary school has been the making of him 
DS2 is 10, he's totally the opposite, he's calm, quiet, hates conflict. He's had medical issues for several years that threw our family into utter turmoil
which were still trying to pick up the pieces of. Life will never be easy for any of us because of it. We also have to deal with the after effects of the treatment- learning difficulties etc.
Subsequently, there's been times when both boys have needed a lot of focus. DS1 found DS2's illness very hard to deal with - he felt we favoured him. I've also, admittedly, spoilt DS1 as I've had to play Mum/Dad a lot of the time and I feel guilty that he's got the life he has.
I hope neither boy grows up thinking I favour either, they just have totally different needs