Most of us keep food that serve as raid or utility buffs, usually from Cooking. This is actually one area that Blizzard still needs to polish, with some foods harder to come by than others. That said, I pack some Dragonfin Filet for those dungeon runs, and a stack of Great Feasts for raids. I would love to bring Fish Feasts, but my Fishing is still at a deplorable 1 (I'm too poor to keep buying the fish off the Auction House).
However, my current favorite food these days are Blackened Worg Steaks, and I never go around without the tracking on. It's an extremely useful buff, specially on a PvP server, so I hardly get jumped and it's very easy for me to track down nasty gankers. I also keep stacks and stacks of Festival Dumplings for general use and because I just love Chinese food. Lastly, I have stacks of Star's Lament because it's the only drink you can take in Arenas.
Blizzard has vaguely said that they'll be adding some new recipes in Patch 3.1, so that should be pretty cool. They haven't disclosed what stuff they're cooking up -- pun intended -- but I sure hope they put in some Resilience food. There are cool, fun foods, too, like the Gigantic and Small Feasts. I hope they put some more silly emotion foods, because those are just darn fun. I can go on and on about the foods of World of Warcraft, but let's move on to our appropriate Breakfast Topic... what foods do you keep in your bags? What are your favorite ones? What foods do you think should be added to the game?