Blue. Royal blue, more specifically, but I like many other shades of blue as well. My second favorite color is probably green, but it comes in quite a ways after blue. I've always had a very strong favorite color preference, and I don't really remember when it started. I've just always gravitated towards blue, and still do to this day.
It's really interesting that some people have strong favorite color preferences, while others don't. When I was younger, everyone seemed to have a favorite color, but a lot fewer adults have strong favorite color preferences, and I'm not sure why that is. I suppose it surprises me because I've always had a strong preference for blue, and it hasn't really shifted at all, so it never occurred to me that other people would lose their favorite color preferences or change them until I started to have conversations with friends about the topic.