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.