Buffy vs. River (Serenity)
I'm going to have to say River on this one. If she went all Miranda on Buffy, it wouldn't be a fair contest.
The Winchester brothers of Supernatural vs. The Buffy gang (minus Buffy, of course, because Buffy could kill them)
I'd like to say the Buffy gang because I think the Winchester boys are pretenders anyway.
Ninja vs. Pirate
The discipline of martial arts against the anarchy of thieves. Hmm, tough contest but I'd have to say the Ninja on account of all the extra weapons they seem to carry and their skills at jumping backwards up to higher places.
Evil Dead zombies vs. Dawn of the Dead zombies vs. I am Legend zombies
I haven't seen 'I Am Legend' (although I have seen The Omega Man, so I'm familiar with the type of Zombies you're talking about). But I'm going to have to add another to the equation: the Zombies from 28 Days Later. Now those suckers are seriously scary and can move so quickly.
Angel vs. Dracula
Angel is a pussy compared to the Count. It is no contest, as far as I'm concerned.
Astronaut vs. Caveman.
Caveman all the way. Astronauts would immediately be inhibited by their puffy suits anyway.
Joss Whedon's shows vs. Gilmore Girls?
Definitely Joss Whedon, although I must admit to not having watched much Gilmore Girls.
This was fun 