At the very start of the Sinestro Corps War when Big Bad Sinestro explains his Start of Darkness, empowers the first yellow power ring and then recites the Badass Creed which will make the entire galaxy tremble in fear:
In blackest day, in brightest night, Beware your fears made into light Let those who try to stop what's right Burn like my power... Sinestro's Might!
The end of that same issue is a crowning moment of terror for the entire series. Seeing Sinestro, Parallax, Cyborg-Superman, Superboy-Prime, and the freaking Anti-Monitor all on the same page sent an Oh Crap reverberating throughout the fandom.
John Stewart and Guy Gardner helping Kyle Rayner and Hal Jordan defeat the evil fear-monster Parallax using The Power of Friendship.
After this, Parallax wants to continue the fight when Ganthet and Sayd show up with the power batteries of John Stewart, Guy Gardner, Kyle Rayner and Hal Jordan, split Parallax in four and seal each part of him in a power battery.
Power Girl threatens to turn Superboy-Prime into Supergirl-Prime with her heat vision.
This also gives the Anti-Monitor a Crowning Moment Of Awesome when he survives after having the freakin' Warworld dropped on him.
The mere fact Guy managed to make the Anti-Monitor(you know, the guy who functionally destroyed the previous multiverse) have an Oh Crap expression is a moment of awesome in itself.
Guy is on the verge of death due to Despotellis, a sentient alien virus who has killed billions in Sinestro's name, killing him from the inside when Guy is injected with Leezle Pon, a superintelligent Green Lantern smallpox virus. The microscopic battle isn't depicted, but when Guy coughs up Leezle with the remark "I think the good guys won", you can't help but cheer for the little guy.
Green Lantern Sodam Yat, an alien with the same powers as Superman, gaining the power of the willpower-entity Ion and dueling Superboy-Prime.
And before that, at The Battle of Mogo, when he blew up Ranx, the sentient city, from the inside.
Genius Bruiser Kilowog can't bring himself to kill his Evil Counterpart Arkillo... so he drops a damn AIRCRAFT CARRIER on him and cuts off his ring-hand instead.
The citizens of Coast City have a Crowning Moment of Awesome after Hal Jordan asks everyone to evacuate the city before the impending battle. No one leaves. Instead, they shine green lights from every window in support of the Lanterns. After the conflict is over, they receive the much-deserved moniker "The City Without Fear".
The speech when The Guardians of the Universe joins the fight against the Anti-Monitor.
"Murderer of worlds. The last time you so violently assaulted this universe you entrapped us in a stasis beam. You besieged us with a fire that burned our souls. You rendered us ineffectual and absent from your last war. Today, Anti-Monitor... The Guardians of the Universe are absent no more!"
When you get an Oh Crap reaction out of Superboy-Prime, then you know you've accomplished something special. So rest in peace, anonymous Guardian of the Universe.
Superboy-Prime: Help! Somebody help me!
Even more impressive is if you get him stuttering and scared by your presence alone.
Superboy-Prime: I d-don't like Speedsters!
The absolutely epic final brawl through the streets of Coast City (with Hal and Kyle de-powered), where every Hannibal Lecture Sinestro tries is met with a Hot-BloodedShut Up, Hannibal!. And there's the end of the fight where Hal smashes Sinestro's head through a window and leads him away with the words "You're under arrest."
Sinestro gets one after being de-powered:
Sinestro: Come on, then. The both of you. *runs right into them*
Near the beginning of the Sinestro Corps War, Bedovian, the Sinestro Corps sniper, starts picking off Green Lanterns left and right from three sectors away, leaving pretty much the entire Green Lantern Corps in a panic. While Guy is trying to get the rest of the lanterns to safety, John Stewart locates Bedovian, constructs a sniper rifle and returns fire. After a satisfying Oh Crap from Bedovian, we don't hear from him again for the rest of the story arc.