Like others, getting out of the test chanbers in Portal just made the game exponentially cooler for me (I still would have loved just the test chambers).
The moments just after the resonance cascade in Half-Life when you wake up in the ruined test chamber. Just knowing the proverbial **** had hit the fan and that I was now going to have to find a way out of the large, imposing Black Mesa facility just filled me with glee and a sense of awe.
When you first crash-land on the halo in, well, Halo. Just looking around the immediate area I knew I was in for an epic journey.
However, the most "forced me to smile" moments I can recall would be pretty much any moment in Half-Life 2: Episode 2. That game just felt right from the opening scene. The citadel exploding sending out the "portal storm" shockwave, getting to drive the busted up muscle car you find on the bridge, meeting the advisors and hunters for the first time, watching dog beat the crap out of a strider, fighting the striders with the new magnusson devices, watching the rocket take off, and lastly watching an old favorite character meet a dramatic end. I don't think there was a moment throughout where I didn't feel a smile cross my face (except that end scene, that was more of a wide-eyed, gaping mouth WHAT THE **** moment)
There are other moments from other games I've played, but I can't recall at the moment.
