Trixie, hands down. She has nothing to lose and no sense of self. Cissa would be too worried about protecting her looks etc.
2. Remus "Moony" Lupin vs. Fenrir "Will Eat Your Children" Greyback
Lupin. I think he too has very little to lose, although he may not feel that way now that he's hooked up with Rainbow Brite. He also has the flame of self-righteousness on his side.
3. Gilderoy "Blonde Ambition" Lockhart vs Ludo "The Wasp" Bagman
Wouldn't that be awesome. It would be like the ultimate "slappy" fight. Bagman, I would have to guess would win. But it would still be funny to see.
4. Severus "Don't Call me Snivellus" Snape vs "Seriously" Sirius Black
Both guys with tremendous chips on their shoulders, anxious to prove something. This would be the most intense matchup. I have to say Black though. Neither of them is above fighting dirty, but he's particularly crafty and the only time Snape ever got anything over on him was when he was distracted with holding off Lupin from Harry.
5. Neville "Stubefy" Longbottom vs Peter "Pathetically Pitiful" Pettigrew
Pettigrew is a pretty good wizard, for a coward. I gotta think he'd take it, but points to Longbottom for enthusiasm and grit in the face of superior talent.