Watching a destroyer or anything of similar size explode. Although you might want to stay clear, since large explosions can damage you. But if the exploding ship in question is one of your allies, well....
The SOC Loop mission where you escort the NTT Sunder. After killing the fighter wings you and Snipes betrayed, more NTF fighters arrive to provide reinforcements. A few minutes later, two wings of the prototype Erinyes heavy fighter warp in and decimate the rebels.
The very first mission of FreeSpace 2, "Surrender, Belisarius!". You start off escorting a Vasudan convoy that is under attack by NTF fighters. Then suddenly, a Vasudan Destroyer of the new Hatshepsut-class, the GVD Psamtik, arrives to assist in destroying a rebel corvette, the NTCv Belisarius. When the corvette refuses to give itself up, the Psamtik blows it away with its Beam Cannons. Just this mission alone lets you know that things have changed a lot since the Great War.
The espionage mission towards the end of FreeSpace 2, where you and your wingman are scoping out the Shivan staging area in a pair of captured fighters. The instant the mission starts, your wingman is screaming in your ears: "DIVE! DIVE! DIVE! Hit your burners, pilot!!" Turns out, you've just jumped out of subspace right in front of a five-kilometer-long juggernaut that's on its way in.
Anything that involves Commander Snipes (the aforementioned wingman) is pure gold. Like the second GTVI mission where you're both in deep cover as NTF pilots escorting a secret cargo to a rendezvous, only to slaughter the other NTF pilots so GTVI can steal it. If you pull it off without losing your jamming cover, Snipes wryly notes "We're pushing our luck here, but what the hell." He then proceeds to wait for the NTF cruiser you were supposed to hand the cargo to, chews out the captain for not getting there sooner and pins the whole debacle and the loss of the fighters he killed on the cruiser's crew. And it works!