Jake and Sherry's campaign. All of it, being a blend of the Resident Evil 5 style and the original games but one moment stands out in particular— Here's the setup. Jake is separated from Sherry. He's lost all his weapons, he's on a narrow catwalk over a pit of lava, and he's cornered by the Ustanak: the monster that has chased Jake and Sherry throughout their entire campaign. The same monster that a team of four armed fighters or even a giant drill to the chest couldn't even damage. Without hesitating, Jake takes it on in hand to hand combat and delivers a No-Holds-Barred Beatdown to it. And WINS.
That final Megaton Punch must be the most awesome scene of the game.
More than just that, the escape from the research facility in China, surviving a huge tank near the exit and getting into one of the most epic motorcycle chases in video gaming, Jake pulling off several perilous jumps and Sherry conducting a running gunfight with their pursuers all while an attack helicopter stalks them through the abandoned and ruined streets of Lanshang.
The fight with a mutated Derek Simmons in Leon's campaign.
Helena's personal crowner comes when Simmons shears off part of the bullet-train's metal roof and flings it towards her. She doesn't even flinch as it misses her head by only a few inches, keeping her focus and aim entirely on him the whole time.
Ada earns a big Crowning Moment Of Awesome with her battle with Carla: the majority of the fight is her outrunning the mutant woman throughout the ship, but she puts up quite a fight, and eventually finishes her off by blasting a rack of liquid nitrogen containers, causing Carla to freeze and shatter into several pieces due to the impact of the explosion.
Ada: It's about time you shut up now, Carla.
During the final battle of Chris's campaign, Piers gets his right arm impaled and then crushed by an attack from the Haos monster. To save Chris, he rips what's left of his arm off so he can inject himself with the C-Virus syringe he picked up earlier, which regenerates his arm and allows it to shoot freaking LIGHTNING. After disabling the Haos, he makes a final Heroic Sacrifice by choosing to stay behind in the collapsing underwater facility instead of leaving in the escape pod with Chris, since he was slowly losing control from the effects of the virus. And with his last act, he finishes off the Haos with a giant bolt of lightning before it can reach Chris's pod.
Although it happens in others, Ada looking out for you in Jake and Sherry's campaign is pretty awesome.
Sherry Birkin coming back after being missing since Resident Evil 2 and originally established as being kidnapped by Wesker in his report. And she can now take care of herself pretty well having Taken A Level In Badass and become a relatively competent government agent who can perform wrestling takedowns against enemies twice as big as her.
That was retconned into Wesker just having spies watching her.