I'm going to have to go with chocolate. Besides the fact that Chocolate is my favorite flavor of all time, I'm going with the fact that Vanilla tends to seem bland to me. Vanilla cake would mix well with a variety of things Just Like vanilla Ice cream.
I like chocolate also for the reasons ALIT posted.
I don't like cake really. Usually gives me an upset stomach. Donuts are the same way.
If I do have cake, I like both flavors, but I would go with vanilla.
It's such an underrated flavor. It's very classy though. The finest of the flavors.