Nemesis (Resident Evil 3)
Why you wouldn't want to meet it up a dark alley: Well, it's a humanoid bioweapon created by infecting a Tyrant with a parasitic organism designed to increase its intelligence. It can regenerate its tissue and, like a monster designed by Homer Simpson, has a rocket-launcher built into its arm.
Movie monster we'd like to see it have a fight with...
Pinhead (Hellraiser, 1987)
Who would win? Pinhead would transport the Nemesis to Hell where, umm, bio-implanted rocket-launchers don't work properly. He would then summon a gang of Cenobite acolytes to systematically cut the Nemesis into little quivering cubes while he stood and calmly observed in his fetishy Goth greatcoat.
He would then, as he says, have an eternity to know its flesh - having the regenerating Nemesis bits for his tea every night forever.