ground beef (93% lean beef preferred), bacon....or more!, Cheddar cheese, Fried onion strings (I think French's is the most common), Lettuce, BBQ sauce of choice. (I like sweet and spicy), Mayo ...
The Straight Sixes Spicy variety gets its heat from ancho chilis and chipotle peppers, and is excellent on barbecue chicken sliders ... burger is topped with pulled barbecue brisket.