Crosby vs Ovechkin would be the most fun, but I still think a better, more evenly matched fight would be Crosby vs Giroux. I think either of those two would own Ovechkin but would have a good fight with each other. Crosby has the lower body strength, Giroux would probably have the meaner disposition.
The only Captains that could give Thornton a run for his money are Weber and Chara.
I would say Iginla, Backes, Staal, Ladd and Phanuef would be good bouts but I'd put my money on Jumbo everyday. We already know what happened with Getzlaf
Now if Lindros came back that's a different story.
I'd bet on Iginla and probably win 7/10 times. Backes could probably give Jumbo a pretty good bout too. Not to say that Thornton isn't at top 5 fighting captain... perhaps even top 3.