The Gears of War 3 beta is currently live. Lizard Dude and I have been playing it, and I decided Gears 3 contains the most pants-pooping-est weapon ever.
It's called the Sawed-Off Shotgun, and here's why I soil my pants with each and every pull of the trigger (no, really):
It's a very powerful but very close range weapon. Anything within arm's reach is a guaranteed insta-kill, including multiple enemies at once. But anything farther than point-blank will miss. The reload time is the longest in the game, and each clip has only one bullet (you get 4 bullets, or shots, total). So basically, whenever you shoot, there are three outcomes:
1) You kill the guy (and possibly other guys) in a glorious burst of red gibblet explosion. This looks and feels a lot more awesome than the poop it causes in your pants.
2) You miss completely, meaning you instantly have to start dancing around trying to dodge / run away from the guy you failed to kill, while you wait for your gun to reload. Either he will own you instantly, or will take you down while you run away. In rare cases, you can escape narrowly. All cases are very scary and thrilling, thus, poop in the pants.
3) You and your enemy walk up to each other and kill each other with the Sawed-Off at the same time.
A final result can actually be a result of either of the first two options: you jam your gun on reload, which extends the reload process even longer. Death is inevitable, and you have to dance around even more. Quite scary.