On the topic of buns.....one of the best buns comes from BreadBar in Los Angeles. BreadBar makes an OK burger, but the bun is great--Wolfgang's Steakhouse also uses that bun. Another good bun, but a WAY OVERPRICED burger is at Charlie Palmer's DG Burger at the South Coast Plaza mall. The DG Burger bun is a chewy Semolina/Potato bun.Semolina flour is what makes pasta tender. It comes from a softer form of wheat than the wheat used for most breads.
A good bun should be firm in the hand but yielding to the tooth. The bun should stand on its own in the bite--this is accomplished by kneading the dough to the point where the gluten strands are long enough to create chewiness. Over-manipulated dough creates a tough mouth feel, and when this happens, the bun is the last thing on the palate. The beef is the last thing that should linger on the palate after each bite. Conversely, under-manipulated dough results in a grainy, mealy, or flaky bun, which allows the bread to blend with the beef--this mutes the umami and beef notes of the dish. This is often the shortcoming of Brioche-style buns.
A good bun should be mildly sweet to provide a counter to the salty flavors. It should be a little yeasty (to add umami flavors).
Buns should be moist so as not to rob the burger of juiciness, but they should be absorbent enough to keep the juice off of your fingers. Naturally, the bun should not overwhelm the burger in terms of volume. The Brioche bun at Olives in Las Vegas nails the both the meat to bun ratio and the absorbency factors.
Buns should be toasted on the cut side--this creates texture and the oil used when griddling the bun keeps the cheese from binding to the bread. It also heats the interior of the bun to retain heat in the dish. This also helps to further melt the cheese, and the steam from the bun combines with the cheese to create a better mouth feel.
Buns should never be scorched--this infuses the entire dish with the taste of burnt starch, and that ruins the dish.
Buns should not have their tops oiled in any way. This is just unpleasant to hold onto.
Seeded vs. Non-seeded--personal preference rules here. Sesame seeds provide texture and sweetness, but they neither make not break the dish.
Finally, buns should be paired to the burger meat. Saltier beef requires sweeter bread. Finer grinds and fattier blends require softer buns so that the texture contrast is not jarring.
Buns don't need to be fancy, and they shouldn't be the star of the show. A bun is a beef delivery device--it should quietly and competently convey the beef to your mouth and lend subtle flavors to the dish.