Without a doubt it would be
Nicky's in Wrentham, Mass. The burgers are Old School; no frills, nothing fancy but... high quality ingredients, bakery buns, everything expertly prepared with the lettuce, tomato and pickles served on the side. The meat to bun to condiment ratio is perfect; everything is always perfect. It is quite simply the best diner in the Universe! I have no connection with it whatsoever but check out this menu:
http://www.nickysfamilydining.com/lunch-dinner-menu/