With a purple dildo (at least in San Andreas)
The Alien Blaster leaves a prettier puddle of goo behind.
Another thing that doesn’t get old: “Reverse stealing” activated bombs/grenades/etc. on NPC’s in any of the 3 Fallouts.
Punching the thieving little kids in the Den in Fallout 2. It makes them run away from you for the rest of the game, and since you don’t kill them, you don’t get the “Childkiller” “perk”.
The Groovitron from Ratchet and Clank.
One-punch knockdowns.
In Fight Night: Round 4, most knockdowns are still done in the usual way: you whittle down your opponent’s health so that he’s more susceptible to being stunned, you land a solid punch that winds up stunning him, then you have 5-10 seconds to do X amount of damage in order to knock him down before he recovers. Sometimes, though (the game must have some sort of critical hit chance modifier), your opponent will have plenty of health, you’ll dodge one of his punches and throw a well timed hook or uppercut counter that lands flush, and the other guy will immediately crumple to canvas. I bust out laughing every single time.
Satchel charges stick to people, in Saints Row 2.
Carving your name into your opponent’s [del]face[/del]base with multiple particle cannons in Command and Conquer: Generals.
It hasn’t had a chance to get old yet, but I imagine that sending about a dozen dead turrets through a perpetual motion cascade in Portal will never get old.
From older games: The gentle “poof” sound that Soulless make when they die and collapse in a pile of purple dust, in Myth II
And in Scorched Earth, killing the entire field of AI tanks with a single shot from a non-offensive weapon. Or even doing it without firing any weapon at all.
Double headshots with the Scout. Always good for a flood of “wtf!” chat messages.
A really well-timed shock combo. A mid air rocket hit. Just a perfect moment.
Finding a fountain of youth in colonization.
Gettin a Monkeylord into an enemies base.
I used to love boxing games, but stopped playing them because they got away from one-punch knockdowns/outs. It was all about whittling down your opponent. I might look into FN:R4 if that’s not the case anymore.
I’ll add: semtexing or fragging an opponent when they go for their care package in MW2.
And as of today: roping someone who has a bounty on their head and dragging them behind your horse in Red Dead Redemption.
Or dumping a load of napalm on a tank in a narrow, steep valley. It always struck me as rather sadistically enjoyable. As was a lot of Scorched Earth, actually - that game kicks ass to this day.
For that matter, there were a lot of exploitable bugs in Scorched Earth, too, some of which were a lot of fun. Personally, I’m a big fan of flying tanks. Or, one you doubtless enjoyed, filling up the inside of a tank’s shield with napalm.
Placing a Supermonkey, and watching the balloons just fall back to the entrance.
Crippling but not destroying the last enemy until you’re able to build a fleet of the largest battle cruisers the game allows.
Flamethrowers.
Boom! Head shot!
The Monty Python foot that squashes you on death in Jet Set Willy II.
Ring
Amulet
Bashing a car up in GTA 4 until it’s just about to explode, driving it full speed headlong into traffic and jumping out at the last minute.
Miniguns (any game).
BOOM!!! HEAD SHOT!!! (especially if the head in question explodes).
Placing a remote charge on an aircraft in any Battlefield game. And then blowing it up mid-air while it’s rightful owner tries to use it.
Killing zombies with a sledgehammer in Dead Rising.
Re: Scorched Earth, the game lives on in Gunbound, if you’re willing to tolerate the insanities that come with a Korean online game. It’s great fun.
For me, Mayhem missions in City of Heroes never get old, especially if you have a strong area-of-effect character. Running into a parking lot and destroying every single car at once is nigh-on therapeutic.
Alternately:
Moo moo moo Moo MOOOOO!, preferably with Corpse Explosion or Death Sentry.
(BTW, there’s still a few of us playing, if you want to join up with us again)
A well placed grenade in FPSs. You throw then 3-4 names all come up at once… 
Rikku’s “stripping” scene when she becomes a permanent party member in Final Fantasy X.
Some things don’t get old for me because I can’t make them happen often enough to get tired of them.
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Heroic victories in Rome: Total War, but not the kind where I run cavalry around until the other army dies of exhaustion. More like when a horde of Gauls attack my small-ish Julian army and I have a nice high hilltop with some cover right next to a corner of the battlefield. Bonus points if I get the historic battle marker.
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Successfully rushing a real PITA warmonger like Montezuma in Civ IV during the ancient period. I suck at rushing. I’d much rather fight in the medieval or industrial period. It’s nice when I have to rush and it works because otherwise I’ll throw in the towel.
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Hearing the “puzzle solved” series of tones in Zelda games, but only if I figured the puzzle out without looking it up or resorting to trying everything possible.
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Beating Matt in Wii Sports boxing, but I get frustrated trying.
I forgot one.
- Killing Sigmund.