Haruhi would be terrible...extraordinary things like zombies would only exist if she wanted them to. So I'd be allied with the person responsible for the apocalypse. In that mental state it wouldn't be strange if she ended the world along with me at any moment.
Any Servant wouldn't work because I'm not a magus, but if I could get Hero Emiya (the living version), that'd be great. He could probably fight off infinite zombies, would have no reservations about executing any infected people, and would do anything to save the world. The only problem is he doesn't care for individuals, so I'd probably be left to fend for myself while he's out saving the world.
