caramel or vanilla. have never heard that milkshakes were regarded as a traditional breakfast drink, I always thought full cream milk nothing added was the breakfast milk. and better for you.
I'd say it woukd be American as the word "mom" is used. For us Aussies it's not a breakfast drink whereas shakes are now being put on the menu as an alternative to breakfast. My favourite was chocolate until I was made a special avacardo milk shake by a Chinese girl serving at a local club bistro. It tasted wonderful.
I only have milkshakes after I've donated blood at the Blood Bank! I use to have lime, but they don't have that anymore, (I wonder why?!) so I now have chocolate. Next time I go, (in Dec) I'll ask for a caramel 1.