GameSpot's Oh Snap! game emblem, plus a recent experience in Batman: Arkham Asylum has made me think about the times I can remember experiencing the equivalent of Oh Snap! moments in gaming. As usual, this is not an exhaustive list; it's what I can remember off-hand. The fact that these are the specific moments I can remember implies that these are the Oh Snappiest! of my Oh Snap! moments. What about your Oh Snap! moments?

Batman: Arkham Asylum: The third Scarecrow sequence. Rocksteady has tapped into the darkest fear of gamers.
Bioshock: Would you kindly? (Even though I had spoiled nearly the whole game for myself before playing, it was still awesome.)
Dante's Inferno: When Dante enters the City of Dis on top of Phlegyas.
God of War II: The opening fight with the Colossus.
God of War: The fire sacrifice. Never have I felt so guilty over completing an in-game task.
Mortal Kombat: Sub-Zero's fatality. OMG!
Okami: Restoring life and light to the world in an explosion of colour and nature.
Pilotwings: Skydiving for the first time. Mode 7 FTW!
Portal: Pretty much the whole game.
Shadow of the Colossus: The colossus battles (maybe not the smaller ones).
Soul Blade: The CG opening with theme song. Whoa.
Street Fighter II: Playing a game with eight selectable characters.
Super Mario World: The first 16-bit game I played. The colours! The sounds! The size of it!
WipEout XL: The banging electronic soundtrack and visual design. Sweet.
WipEout Pulse: The MagLock strip on De Konstruct: ships zoom by overhead and upsidedown!
Wolfenstein 3D: The first awesome 3D shooter.
Doom: The first really awesome 3D shooter; and, Encountering the Cyberdemon.