Who would win if a black widow spider fights a tarantula?

It depends on which species of snake are you talking about. If it's is a Cobra, King Cobra and Brown snake, a black widow doesn't stand a chance. Most snakes, however, are not venomous at all.

P.S. A black widow isn't the most venomous spider, there are also the red back spider and funnel-web spider who are much more dangerous.