engineer, one of the biggest problems would be getting supplies easy and safe, for that i have teleporters, running out of ammo or getting hurted, dispenser, safety while sleeping, sentry.
heck i could just sleep all day and once in awhile take a teleporter to the mall and stock up on food and if i was bored i could just shoot a couple hundreds zombies.
edit; to all of those who haven't chosen engineer, imagine this, you would get no rest what so ever,and even if you did you would tire down for having to carry and use your weapon at all times, also healing, sure scout could out run most of them but if he ever took just 2 hits he would die, and if we are allowed to rp beyond the rules of the game, engineer could make entire citys safe and rebuild civilization, and you would never have the moral dilemma of killing a loved one who got the virus, the sentry's could take care of that as soon as they turned "evil".