Zombies by far. People say they're the easiest to beat, because of missiles and machine guns? A single missile won't do much good when you globally have almost 7 billion -potential- to-be zombies walking around (or running around in panic only to get themselves killed and turned). As for machine guns, that sucker is gonna run out of ammunition at some point, and most people who got themselves weapons and ammo are going to get themselves eaten or turned anyway, so when you find a box or two of neat M249 ammo, it'll be right inside what can only be called a "Zombie Lunchbox".. In the case of a zombie scenario, it'd be realistic that 50% or more of the worlds population would be turned within a week or less, as they'd get themselves killed by panicking and doing something stupid like getting themselves cornered, or... fight zombies with their bare hands just to.. prove something. (Also, with most humans living in densely populated cities, the human centers of society would quickly become the worst possible places to be)
I also assume that we work by the "one bite/scratch = turn" policy, and also that anyone who dies will eventually wake up as a zombie, bitten or not, which would make you very wary of anyone who might be dying of less unnatural causes. *Glares at the old man with the failing pacemaker*
Vampires are dangerous because of their intelligence, so making them unintelligent makes them sound more hilarious than scary. (I take unintelligent as in "Beat head repeatedly with a wooden plank for fun" sort of unintelligent) Also I prefer the 'World of Darkness' (Vampire: The Masquerade) kind of Vampire over any others, and those are not affected by garlic, crosses or stakes (That is, stakes paralyze them temporarily if hit in the heart, but don't kill them) TheFurryChicken pretty much summed up the rest of the reasons to why they're not as bad as zombies.
Werewolves.. I'm gonna go with the World of Darkness kind here too, they don't act sentient but sure as hell don't have many weaknesses. But seeing how they mangle a victim to the point of beyond recognition I don't see how a major outbreak could happen. The World of Darkness werewolves are "500 pounds of hair and claws, bullets don't hurt them and getting close is suicide". I don't know anything how silver bullets work on them, or if anyone has ever gotten to try, but in that universe, a single werewolf can wipe out a whole group of vampires, so even the vampires fear them there.
