The final boss should get a mention. Not only is it Jack's most evenly matched opponent, but the fight is against none other than the Black Baron, AKA the guy who gets mangled in each level by his own girlfriend. Suddenly he gets serious and shows exactly how tough a guy who keeps coming back from death can be. The kicker is his taunt to Jack right before the fight.
"Navy Seals? Special Ops? Come on, bitch!"