I'd say vanilla (in milkshake at least). I don't like strong vanilla flavour much though, ever since my labcoat got covered with vanillin (as part of chemical purification practical when I was a student). Man the smell lasted for weeks, my bag stunk of it, couldn't get the smell out. Vanilla pods are cool though.
Back on topic, on the chocolate note though, the way to argue with a vegan is ask them is carob really a good chocolate substitute? I was vegan for ages and never learned to like carob, even if i did it isn't like chocolate (or cocoa I should say). It's pure filth. Might not be a fact but personal experience, I'd rather eat soil hehehehe.