There's always a lot of activity on game soundtracks, but what about sound effects initiated by player actions? I suppose you could talk about ambient and background sound effects too if you really wanted to.

For me, the sounds of the shotgun in Resident Evil games, the crossbow in the Half Life series, and the charge shot in Metroid Prime games have always been close to my heart. Oh, and the sound of the medic's medpack blipping as it infected a hapless enemy sniper in Team Fortress Classic has brought me tremendous amounts of joy and glee :I

The terrifying clinking of grenades.
The almost gleeful cranking of a chainsaw.
The satisfying thump of a shotgun blast.
The awe inspiring firing of a tanks main cannon.
But my all time sound effect would have to be Star Wars blasters and such, don't believe me?
Play Battlefront or its sequel for more than an hour and when you stop playing you WILL still hear the persistent firing of multiple blasters.

The wet slap made by most of the melee weapons in Condemned and Condemned 2. Satisfyingly brutal.
It also features some great ambient sounds like the rattling of discarded cans, distant footsteps and the occasional barking dog or remote gunshot.

The first time you jump, squash a goomba, grab a coin, grab a power up, fire your first fireball, the enemies die running into said fireball, grab the star, earning your first 1-up, hear ground theme, go into a pipe, hear the underground theme, hear the water theme, hearing the game over jingle, hearing the level complete jingle, hear the fireworks, lose a life, hear the flying Cheep-Cheeps and Bullet Bills, meet up with Lakitu, finally defeat Bowser. Nothing compares to the original Mario Bros.

Grenade launcher in Duke Nukem 3D
Fission Cannon in Turok 64
Explosions in Half-Life 2
Racoon Mario spinning his tail
Background Ambiance in Banjo-Tooie on the beach level

danosaurus:
Grenade launcher in Duke Nukem 3D
Fission Cannon in Turok 64
Explosions in Half-Life 2
Racoon Mario spinning his tail
Background Ambiance in Banjo-Tooie on the beach level

This is a fine gaming pedigree to espouse!

The earth-rumbling *whump* of dropping a Colossus in SotC.

The Bouncer in the Ratchet & Clank games is also pretty satisfying.

Lighting up your lightsabre in Star Wars game (Mostly in KOTOR though).

Charging up the X-Buster in Megaman X
The sound of movement and selection in menus in any Final Fantasy

I really like:

From Diablo II
-Sorceress' spell Thunderstorm in Diablo II. I dunno why, it just seems dangerous.
-The Barbarians main three warcrys (Shout, Battle Command and Battle Orders) are all really sweet too.
-The Paladin's aura Redemption

Warcraft III:
-Bloodlust. Don't exactly know what the sound is (growling? and incantation? growing pains?) but it's super exciting and intimidating.
-anything the Tauren Chieftain, Lich, Dreadlord, or any Troll says is wicked.

Basically all the sound effects in Blizzard games are top notch.

Unreal Tournament:
-all the soundbites from computer characters are amazing (especially "MY HOUSE!"). The single player arena play in that game was made terrific partially by the awesome AI, but equally that their taunting was neat (I want to say realistic, but most online taunts are made by like, 10 year olds using words they don't know the meaning of).

I'm sure I'll come up with more, but I'll let you digest those for now.

The "Crack" when you land a headshot in gears.

It was a long time ago but there was one of these before...

Lighting a field of grain on fire in Overlord. One with a lot of halflings in it. Nothing beats those screaming voices of pain. Hehe...

Tink, tink, tink...

In CoD4, nothing like hearing this sound. Especially on HC servers.

DreamKing:
The first time you jump, squash a goomba, grab a coin, grab a power up, fire your first fireball, the enemies die running into said fireball, grab the star, earning your first 1-up, hear ground theme, go into a pipe, hear the underground theme, hear the water theme, hearing the game over jingle, hearing the level complete jingle, hear the fireworks, lose a life, hear the flying Cheep-Cheeps and Bullet Bills, meet up with Lakitu, finally defeat Bowser. Nothing compares to the original Mario Bros.

I read your post and every single sound was in my head. Thanks.

To contribute, I loved the weapon sounds in FEAR.

The sound of the Half-Life 2 Pulse Rifle.
*D-D-D-D-D-D-D-D-D-D-D-D-D-D* (Wergwurg click beep) *D-D-D-D-D-D-D*

Metroid series in general but my favorite has gotta be the nova beam from Metroid prime 3 corruption.

E.v.a.n.g.e.l:
Lighting up your lightsabre in Star Wars game (Mostly in KOTOR though).

Don't know how many times I hit Y on the controller just to see you light it up and twirl it around.
Slicks on forza sounded so real, heck everything about that game sounded/felt awesome

Najos:
Tink, tink, tink...

In CoD4, nothing like hearing this sound. Especially on HC servers.

That is EXTREMELY satisfying.

The shotgun in most FPS games. Especially the rare double-shot variety.

The noise the Star Dragons make in Star Fox 64 when they're destroyed (sounds like rapid-fire lasers, only cooler).

The screech of a Metroid... unless you're on the verge of death, then it's alarming rather than satisfying.

Anarchemitis:
The sound of the Half-Life 2 Pulse Rifle.
*D-D-D-D-D-D-D-D-D-D-D-D-D-D* (Wergwurg click beep) *D-D-D-D-D-D-D*

They got that gun so right.

Half-Life 2 pulse rifle when you use it. When someone else uses the rifle, it makes farting noises
Best sound effects for me so far have been in Diablo II. Every weapon is so satysffing to use. From swords slashes to hammers blunt attacks. Beautiful

The sound of shotguns in general, it may be have been said but...
...the satisfying blast of what you know is your enemies impending doom...
...the bone-crunching snap of hearing (wait for it) EVERY bullet land from the shot (if your lucky)...
...and not to mention, the pump-action reload...
...only to start it all again
Warm's my heart really...
Few games that include shot-guns have messed this up, well not on my list anwyay

Any gun noise that makes a *thuk* noise when it fires. Either that or any flamethrower noise.

Also, the sound of a dirtbike starting up in GTA.

The clank noise of the robo arm in Bionic Commando.

There's only two things that carry my ears to noisy sound heaven:

from the Unreal Tournament Series: the sound of your flak cannon shoved up your enemies' nose and turning his head into a red unrecognisable projectile.

from Crysis: a silenced SCAR in single-shot whenever you managed to take out another of em korean buggers from long range (moreover when nobody appeared to notice where it came from)

Reloading+shooting grenades from a launcher. I love that sound.
Shotguns. Satisfying POW.
A Falcon Punch connecting on a 150%+ enemy on any super Smash Bros. game.
Gravity Hammer from Halo 3. always found that one particularly nice.
Oh, and the Halflife 2 headcrab zombie scream when they're on fire. Makes me feel satisfyingly sadistic.

Grinding that perfect rail on skate ^.^

The muffled CRACK of a zeller, sending the enemy flying 10 feet with a shot through his head.

The SSR4000 sniper rifle from agent under fire: loud, powerful and satifying. Even better when followed by killing someone.
M203 and from CoD4 makes an amusing pop. Even better when followed by killing someone.
And also the ricochet of a head-shot in CoD4. Even better after killing someone.

Pretty much every Critical hit on FF7.

Unreal Tournament 2003 pretty much every sound the Announcer makes when you get a head shot or a number of kills in short time.

Detonating 8 stickies as a Demoman on TF2 Insta-gibbing 3+ people, then happily taunting, "Ka-Boom"

Another for TF2, getting a revenge kill (with a Pyro taunt)

Using a Stimpack on StarCraft then listening to your marines as they begin to pop some 50+ Zergling's

Pretty much every hit with a rusty blunt object to a crazed hobos head......in Condemned....not real life....i'm not crazy.

Spies BURNING TO DEATH. Burn ,baby, burn~!

The oh-so-satisfying "Plink!" you hear when you land a headshot while sniping in COD4.
All the gun-sounds from that game are near perfection too.

The crunch from whenever a rag doll hits something hard, especially if it's head first.

There was a Star Wars flying game based on Episode 2. The bombs from the SLave 1. Duuuude thats sweet. Wuboaaaaaaaaaaaaww!