The entire assault on the wedding in the Season 5 finale is made of win.
The most notable part is the end. Just when Trent is about to frag both Arbiter and Claire, Greg returns and pulls a Big Damn Heroes moment.
The original "Wedding" episode in Season 3 as well.
In episode 2 of Season 6, Arbiter saving Chief, Cameron and Cody from Chaos Theosis.
The whole fight sequence at the beginning of "Blackout".
The end of "Remission". After TOSERS was disbanded, all the moderators (including Claire) were fragged, and the network soon to be shut down, Arbiter and Chief make a hit list of all the members of Chaos Theosis and prepare to take matters into their own hands.
"Cradle to Grave": The duo are pursuing Adam, on one of Adam's custom maps made into a racetrack of sorts from earlier in the episode. As they chase him around the track in a rocket warthog, they start arguing like an old married couple over Chief's notorious inability to aim. Arbiter drives them off a ramp at an angle, leaving them tumbling in the air, and Chief proceeds to nail Adam's warthog with a rocket while upside-down and spinning in mid-air. Even Arbiter had to acknowledge how badass it was.
Adam's mom and friends standing up to him, helping him get arrested. Give bonus points to the guy who verbally flipped him off, after finding out Adam had the power to ban people while at gunpoint.
Adam's defeat in "Cradle to Grave" is a mixture of funny and awesome.
In "Severance", Chief gets one when he manages to outsmart Kylie and frag her using hologram. CHIEF.
Same episode Clyde is talking over Skype with Kylie after she was fragged. When he looks up, he sees Arby and Chief looking at him with their weapons raised. They waste him in a second. Also Arbiter expecting Chief's betrayal and turning off their hacks before they enter the game.
Chief saves the entire OMN by accident by running over Trent with a tank, not even knowing he was there.
Tyler getting mauled by his long suffering dog, Charlie.
Tyler: AGH!!! MOTHERFUCKER! GET OFF ME! GET THE FUCK OFF ME GOD DAMMIT!!! CHARLIE?! AAAAAAH!
Chief posing as Peter in Colin's IRC server, and then giving the details to the police; which leads to the police breaking into Colin's house, which can be heard in the background of his headset, and being forced to the floor before being read his rights.