minigun: Miniguns are bulky and are not very mobile, would only be good if you're defending a place from zombies
flamethrower: a bit bulky, can be hard to find fuel for ones they used in WW2. Homemade can be unstable if you don't know what you're doing. Plus zombies don't die instantly from fire, you'd have to worry about flaming zombies getting too close.
chainsaw: Effective, but slow, not against a horde
katana: Good for decapitating if you know how to use one effectively
gordon freeman: Fun to watch, but would kinda take the fun out of it.
shovel: Not as good as a katana, but much easier to find lying around
portal gun: Extremely unlikely to find, but has the potential for mass amounts of hilarity.
other: So many other options.
From the choices, I think my love of zombie hilarity will take priority, so Portal gun it is!