Psychic duel:
Nick Scryer from Psi-Ops vs. Razputin from Psychonauts
Go Raz!
Dimensional shifter duel:
April "The Longest Journey" Ryan vs. Chell "Portal" Whatzername
An interesting battle; each fighter would struggle to gate the other into an impossible situation. I think April's got the edge because although either of them can pass unharmed through a portal, only April can pass through a shift and survive.
Stealther duel:
Solid Snake (Metal Gear) vs. Garrett (Thief)
Close call here. Snake's tougher and he's got the advantage in firepower, but Garrett is sneakier, and he's got that detatchable eye camera.
Bloodsucker duel:
Blood Rayne vs. Smiling Jack ("Vampire the Masquerade: Bloodlines")
Jack tears her to pieces. He also makes disparaging remarks about her incredibly stupid wardrobe and laughs a lot.