Best burger I had is from a chain in Iowa called B-Bops. I moved to Texas about 13 years ago, but still stop at B-Bops whenever I go back to Iowa. They also have a really good pork tenderloin sandwich.
B-Bop's has won Cityview magazine's Best of Des Moines award in the category of Best Burger for twenty six years in a row!
If you're ever in Des Moines, I highly recommend you try them out.