One food cooking method I wish would come back is french fries deep-fried in actual beef tallow, the way so many of them - including those at McDonald's - used to be routinely cooked. I realize that isn't everyone's idea of a healthful food, and of course it doesn't work for vegetarians. But they were more delicious than just about any you can find nowadays, and probably contributed to the decades-long popularity of McDonald's fries as much as anything did.
(Speaking of healthful, one article in The New Yorker some years back suggested that these would actually be more healthy for you than some of those cooked in whatever they use now.)
Oh--and in that vein, the style of burgers and fries at the extinct Dolores Restaurant -- especially at the original drive-in, though for the first few years following its demise you could still enjoy the food at the Dolores West. These items were unique and exceptional in goodness...and almost impossible to describe to anyone who never had the pleasure. On the subject of french fries, their Suzy-Q fries were, again, better than anything. They were the first "curly" fry I knew, and not like anything you get now with curly fries. No silly spices (which seems to be synonymous with curly fries nowadays). Just the best, most luscious chains of deep-fried goodness you ever tasted. There's nothing I can say here that would do them justice.