For me, it's the Z Fighters from Dragon Ball Z vs the Avengers.
The group from Fairy Tail vs the group from Kung Fu Pana 2...mother of god...
the cast of Dollhouse vs the cast of V for Vendetta...i actually have zero clue how that would go down, sure V for Vendetta has the strength and skill advantage but they'd just fight each other, at least the Dollhouse characters would work together
The Z Fighters vs Michael Moore. DERP :S
So, Doctor Who versus Keiser Soze?? Well, criminal mastermind or not, you're losing that battle Mr. Kint.
the cast of new girl against the cast of pulp fiction? I know who will win (hint: says motherfucker heaps)
My Little Infected: Rage is Magic- The Mane Six vs the infected from 28 Weeks Later. Well Twilight could teleport to safety, Rainbow Dash could fly to safety, and Pinkie Pie can just bend the laws of physics to her advantage. Tough call on how the other three would do though. I know the Apple family sure wouldn't go down without a fight. Perhaps they'd protect Fluttershy long enough so that she could fly to safety. Though if even one pegasus got bit that could spell disaster. I'm thinking Rarity would be first bitten, but not before putting up a fight first.