Okay, I read through most of the threads on the board, and didn't see anyone mention it, so I thought I would. By far the best place for a sandwich in SF is M&L Market on 14th@Market. (directly across from Safeway). OPEN 11AM-4PM Mon-Fri only.
HUGE portions, Cheap Prices, and a friendly, although demanding staff. Run by May, Judy, and Heather, three generations of the same family, who have been there for what seems like forever. Hand made soups have always been delicious as well, although the sandwich is so big, I often can't get both.
Don't plan to eat in the restaurant if you come during prime lunch times.. it fills up fast.
My favorite sandwich choices are either Turkey and Pastrami on Light Rye or Liverwurst on an onion roll.
And Remember BREAD FIRST!