By default, as I only have DD.
My sisters would probably say I was the favourite growing up, and looking back, I wouldn't disagree. My mum would always buy us stuff, but she always seemed to splurge a bit more on me, especially when I was younger with my clothing. When I got older, this tailed off so it was more fair, but she would often get me books when I asked for them (as she liked to encourage it).
Perhaps it's because I was an 'easier' child by my mums words? Definitely when we got older my older sisters were definitely the ones to get into trouble with school (nothing major, just your general popular crowd rowdiness) whereas I was more reserved and studious.
Nowadays, I think my mum is happier when I visit but I think a lot of that is because I've moved away whereas my sisters are still in the same area as my mum, so she sees them more often. She will often go all out and cook for them when I'm not around, and also for me when I visit, so I don't think she has a favourite anymore. To be honest, I think her favourite now tends to be whoever is the youngest grandchild as she loves infants!