When the Asimov goes down, and subsequently when you apprehend the rich arse responsible for it.
IL-2 Sturmovik: No scripted events or gameplay with plot, but lots and lots of cases in multiplayer, playing with real people, when they or you die... or when you fly as a team and fly a really successful sortie, and return to base with several kills each. And the times when you manage to return to base heavily damaged, your field of view covered in oil and blood... or manage to drop several tons of bombs right on target from 4000 metres from heavy flak. I just hope I'll be able to recruit all the people in the comms sometime to fly as one massive bomber formation, with maybe just a few escort fighters.
Red Baron. First time I was assigned to the same squadron as the famous aces (Richthofens, Voss, Göring, Udet, Fonck, Immelmann, Rickenbacker etc...) and realizing that they suck horribly and I'd be better off flying alone since they had alarming tendency to die on my watch - or they collided in me. Resulted in me always flying all the sorties solo whenever possible.
War in Heaven: several [REDACTED]
Grand Prix 2: Managing to set-up the car so well on some track that I could break the records and win races. Also, lack of Senna and Razenberger on the grid.
Interestingly, Mass Effect's choice on Virmire wasn't really THAT hard for me. I had assigned Alenko to the Salarian Task Group team and was already on my way there, and if I ever learned something in the army is that second-guessing and running back and forth in battle zone is not desireable. Add to that the fact that Lt. Alenko was accompanied with several Salarians and that Gunnery Chief Williams was alone, the choice from military perspective was clear. Also, I never was very attacted to Ashley's character in the first place (I never really had any use for her on missions since my Shepard is a soldier, plus I found myself disagreeing with a lot of her opinions and views, so meh).
Mainly, though, I went to save the Salarians. Kirrahee's speech was more awesome than anything Ashley OR Kaidan ever did for me.
Which by the way I am going to include here. HOLD THE LINE!
Another stuff in ME/ME2:
-Fai Dan shooting himself. We had those nifty gas grenades, we could've just put him to sleep...
-Loss of the SSV Normandy in the beginning of the game.
-Tali'Zorah's loyalty mission
-Meeting Liara in ME2 and asking what's the real reason she's so troubled.
-Samara's loyalty mission. Talk about creepy.
-Stag Party group consisting of a Salarian (groom to come), a Turian and a Human watching an Asari exotic dancer. They all thought the Asari looked exactly like their own species
. Think of this for a while if you don't immediately realize the implications of how disturbing this is.
-Firing the Cain Nuke Launcher for the first time.
TIE Fighter, the few missions where your wingman is the goddamn Darth Vader and you're tasked with saving the Emperor (what the hell Palpatine, how did you let them rebel scum to abduct you?)