[SamandMax]

Silent Hill 2
Watching the tape.

Scratches
Finding out the real source of the scratching noises.

Monkey Island 2
The ending. Love it or hate it, it's definetely memorable.

Police Quest 4
Breaking into the home of the crossdressing serial killer. It's intense.

Super Mario 64
The first time you ran around controlling Mario in complete 3D, felt like such a huge leap forward.

BioShock
First time seeing a Big Daddy pat the head of a Little Sister, the golf club scene, the shocking twist in the story. The entire game is pretty much a memorable moment.