- Becoming giant Mario and crushing everything in your path.
- Hacking a health station and watching a Splicer poison himself.
- Smacking a groaner/wormhead in mid-leap with the hammer.
- Listening to Sinistar rant.
- Stapling a Combine to the wall with the crossbow.
- Basking in Auron’s coolness.
- The BFG.
- Fatality!
Popping a zombie head like a zit with a nice head shot
Knocking the sun out of the sky with a Koopa shell in the Quicksand level in World 2 of Super Mario 3.
'Cause, you know, YOU CAN KILL THE SUN!
EDIT: Oh, and world 5-3 from the same game. Stomping everyone with the Goomba Shoe.
The feeling when you solve a very difficult puzzle in an adventure game without consulting a guide or hint book.
Taking someone’s head off and pinning it to the wall with a single shot from the Predator’s spear gun.
Dropping on some poor unsuspecting bastard from above as an Alien.
Going to town with the pulse rifle, the sound of the gun, aliens screaming…
Damn I miss that game.
A lot of these, but headshots. Definitely headshots. Especially with a shotgun. BOOM!
Blowing up cars in the Call of Duty series by sticking them with Semtex or rolling a grenade under them. Collateral damage caused thereby is, well, collateral.
Mowing down pedestrians in Centr… er, I mean Middle Park in a freshly stolen car.
Setting splicers on fire. 
And then electrocuting them when they try to find water to put themselves out with.
Killing splicers with nothing but Insect Swarm and the Invisibility Plasmid. It takes a while, but it’s funny. (Actually, any death involving use of the Invisibility Plasmid is amusing. Except when they actually bump into you and make you visible.)
Throwing six proximity mines onto an exploding barrel/tank and then Telekinesing it into a Big Daddy for a one-shot kill even against Elite Rosies on Hardcore.
(The above is all from the first game. Haven’t played the second.)
Shooting mechs in the legs in Mass Effect 2 so they fly off and watching them crawl toward you very slowly as if they could actually still do something useful.
Crotch shots in the first two fallouts.
Throwing combine soldiers around like ragdolls in the final level of Half Life 2
Beating up hookers in Grand Theft Auto.
Getting killed by a player who knows where all the armor is and thus just takes your hits until you die in the original Halo – but not before you throw a grenade at him and thus blow him up right after you die.
You don’t have to be playing a video game to enjoy this particular activity you know.
A lot of the others do at least require you be playing a video game. 
The chicken gun in “Heretic”.
Putting a whole row of trap spears/arrows along the length of a corridor, then plinking a Bouncer or Brute Splicer with a single shot from the revolver or rivet gun. The face skid makes it all worthwhile.
Likewise, putting a cluster of 24 trap rivets right where you know the Big Sister is going to appear. Some proximity mines and nearby mini-turrets make for added hilarity.
Some more:
Running over the whole line of Hare Krishnas in GTA 1. “Gouranga!”
C4ing tanks & APCs in BF2.
That sound hunters make when you kill them in L4D.
Reducing 30’ tall mutant behemoths to small puddles of glowing goo with your plasma rifle.
One-shotting beasts that used to kill you over and over when you were a lower level.
Taking out anything with a stealth technician in the original Command and Conquer.
Lulu’s lulus.
Wow is Sinistar still around? I haven’t seen that game in ages. I loved it. It was practically orgasmic to to whomp that big face to pieces. Beware, I live!