Having baked professionally over many holidays, the recurrent best seller always seems to be pie. That's always been number one. Other things that sell well seem to be more dependent on the area you're in. In one location, people really loved the decorated xmas cookies. In another, I was able to sell a good number of yule logs. One place I worked had a pretty good demographic of people of Scandinavian heritage and I created build-your-own kransekake kits which included the prebaked almond rings and ingredients to make the icing with instructions. I also included little paper Scandinavian flags (Norway, Sweden, Finland).
I've also made gingerbread house kits with prebaked pieces, candies and a royal icing recipe. They sold surprisingly well.
I've always taken stock of the area I'm in, what is currently selling well, listening to customer comments (usually overheard), and the general demographic. I also check out other local establishments to see what they're making and taking note of that as well.