Maybe not an intense reaction, but a really intense time is the very last level in Halo.
You run out of the elevator, grab a Wart Hog and race to your waiting ship. From everything trying to stop you and all the jumps and obstacles.......it's intense.
Our first attempt, my son was driving and I was manning the gun in the back. Down one corridor, there was an explosion that caused me to lose my grip on my controller and my man jumped out of the WartHog when the controller landed on my foot. Well, it was intense! Or maybe we were intense and the explosion snuck up on us.
We now go back and we both grab the WartHogs and race each other to the end!
I was playing Aliens vs Predator 2 on the PC, those creepy halls and all the dark corners...creepy music. And all you have is a flashlight for a light that doesn't cover much surface area. Anyway a big alien was hiding behind a crate in a completely dark place and pounced on me and made me jump..I've never had that happen while playing video games before! I've never played Res Evil but I've heard it does the same thing to ya.
In it's day, System Shock 2 was great for scaring the crap out of you. It's a very dark game, and lots of things happen that can really surprise you - especially if you play it in the dark by yourself
More recently, I almost jumped out of my chair while I was playing Splinter Cell.. I was in the middle of a level, trying to run through it quickly. I ran around a corner and suddenly something was there, right beside me! I opened fire, but soon realized that it was only my own shadow! Damn those advanced lighting techniques
System Shock: An adventure in aluminum trash collecting. j/k I remember playing System Shock in a dark room w/ lights flickering off the walls from the monitor...spooky. Alone in the Dark was spooky as well. *shiver*
My "laughed so hard I almost hurt myself" moment was playing Doom, original Doom that had the 2d sprites. 2d sprites weren't all that impressive, but it allowed you fight an insane amount of monters at one time on some levels.
There there was room with what seemed like hundreds of the pink bulldog monster things, which was locked.. Instead of the normal creepy sea of low grunts n' growls--I had replaced the normal low grunting noise with 'beavis 'n butthead's patented "Huh...Huhh..huh..huh" laugh. Pretty hilarious to hear it comming from so many pinkies..
HL2 is gonna be cool, and will probably kick ass but I think Doom 3 is gonna scare the pants of people.