Well, since zombification does not affect animals as much as humans, I would want Applejack. She could kick a zombies head clean off, and she is not afraid the get her hooves dirty. Not to mention she is a farmer, she easily get food to prevent us from starving. I don't think there are vary many other ponies that you could rely on for food. You would probably starve to death if you had someone like Celestia or Rainbow Dash.
I never thought of that, that is smart! Still going with Applejack though, it is the apocalypse, if the zombies don't get you, you will probably starve. Not to mention Applejack could kick a zombie to the other side of the planet!
*Edit: And Applejack is awesome!