For me, it's Zaitzeff. Hands down.
Grass-fed, hand ground in-house, using quality beef (either Wagyu or sirloin).
And the muffin, actually a Portuguese muffin, is a great vehicle for the beef even though it's not a typical English one, sure looks and generally tastes the part of its English counterpart.
A close runner up are the sliders from Bibble and Sip, no beef and not really a burger but it can make you forget all about McDonald's Egg McMuffin. Which for me is no small task.