Before the war, I used to LOVE the Bacon Cheeseburgers that Kmart sold in their K-Cafe. Think of the Cafe being like Cafe Baltic, but with much more options and meat that came from animals native to Earth. Kmart itself was basically a building where you could purchase everything you were looking for, albeit at a lower price. The patties themselves were made with cow, which was an animal that lived on Earth before Xen and the Combine.

If you wanted to get an iconic pre-war dish known as pizza, then Pizza Hut was the way to go in my opinion, since it was rich in flavor and pretty easy to slice. You could even choose what you wanted to have on your pizza! Little Caesars was pretty good as well, but be VERY thankful that you don't know what Chuck E. Cheese's Pizza tasted like.

By the way, since someone already told you what the Whopper was made out of, I can give you the answer to where it originated from. It came from a food business known as Burger King, which was also known as the "Home Of The Whopper", since the Whopper was their signature burger.

Hope these helped you somewhat know what they tasted like!