My love for the Double Down has been well expounded-upon, so I need not go into THAT here...
I, too have noticed a decline in quality at Burger King, which depresses me... we never went to McDonalds's, because my parents said that they donated funds to the IRA, or somesuch. Whether true or not, I've always carried that concept in my head.
However, for fast food burgers, my all-time favorite is In-N-Out burger, only available in the west... The closest eastern equivalent would be 5 Guys Burgers and Fries, of which we have one in Savannah, and they're pretty damned good. Problem is, they're outside my precinct, so I can't go there to eat... *whimpers*