The Final Boss fight against Sullivan, especially if you've been working on your hand to hand combat moves and have acquired the haymaker and double leg drop kick. A hand to hand battle against the Big Bad on top of a building surrounded by a sea of zombies while an AC-130 blows holes in the building in the background... epic. Then there's Sullivan's death. How many other villains get ripped in half by a plane!?
Anytime you use the Blitzkrieg. It's an electric wheelchair with machine guns attached that taunts zombies in the voice of Stephen Hawking!
Player created moment: Delivering the final blow to blow up Tyrone King's helicopter with a book.
Tear Jerker moment aside: Frank West taking on Chuck Greene in Off The Record. Both of Dead Rising's protagonists taking each other on to determine who kicks more ass.
Making it better would be to have a co-op player join you for a good ol' Mirror Match.
Somewhat dampened by the fact that Chuck's already gone insane before the events of Dead Rising 2, meaning he hasn't taken any levels in badass. Due to this, his boss fight is more or less the exact same as Leon's.
The final boss fight of Off The Record. To put it into perspective, Stacey knew that she had to deal with Frank West. You know, Frank West, a guy who's covered wars and killed more than 100,000 zombies, insane military officials, Ax-Crazy clowns, corrupt cops among others. So she decided to bring in a freakin' giant robot to kill him. He manages to own it.
A good survivor example would be the band Angel Lust who are found playing a concert, having apparently not even noticed that everyone has become a zombie. After Chuck climbs up on stage to get them to follow him they finish the show by rocking so hard that the crowd of zombies who have gathered below all die from a headsplosion! Then they give you a combo card that allows Chuck to do this!
They also frequently show up right before you fight off TK's mercenary goons robbing the casinos in Fortune City. Give 'em all the dead merc's rifles you have been picking up along the way, and they become your very own elite squad of mercenary-killing British rockers.