The most over-powered weapon/spell/item.

[Supernovajake]

Forgot Sin's overdrive in FFX. You just die. No way out, not even auto life. The attack may as well be called Game Over.

 

[CmdrGoob]

Neverwinter Nights: Timestop
Does exactly what you think it does, stops time for a little while allowing you to cast a number of other spells and have them essentially fire all at once, once the time stop wears off. End result, you'll clear entire rooms in half a second, rip most of the HP off any boss in moments and then you can do it again, since you do have a couple of these memorised.

Neverwinter Nights: Weapon Master (prestige class)
A clever combination of feats and innate abilities allow this class to instant kill anything it can critically hit with its chosen weapon (it's effectively impossible to make the saving throw). That includes things like adult dragons, although you do need a very high character level to pull this off.
Undead being immune to critical hits pose a bit of a problem since you spent most if not all your feats to get that ability.

Neverwinter Nights 1 & 2: Isaac's Greater Missile Storm
Fires 1 missile per character level at a random target within range, dealing 2d6 damage each. This doesn't sound to bad, until you consider that there's no cap to the number of missiles. Above level 20 this spell makes anything easy (level cap is 30 in NWN2 or 40 in NWN). Usually you won't encounter enemies in groups of more than 4 or 5 and you can have quite a few of these memorised.

Neverwinter Nights 1 & 2: Fireball (and elemental variants, Ice Storm, Scintilating Sphere and Horrid Wilting)
deals 1d6 damage per caster level to anything in range. (again, not capped.) try 20d6+ damage to anything in range of the blast and watch it all melt.

Neverwinter Nights 1 & 2: Weird and Wail of the Banshee
simply put, all enemies in range die unless they make the saving throw. You can get a really high Difficulty Class to these spells with the right combination of level and feats. End result, unless it's immune to death magic it's dead.

Ah, NWN. Mmmm, maximised missile storms and wail of the banshee, good times.

Also reminds me of my favorite overpowered character - a dual kama wielding monk (1)/ cleric (20+)/ fighter (6, added in epic levels) with high dex and wis for an awesome AC, up to 10 attacks per round using flurry of blows and divine power, 25+ damage per attack thanks to various cleric buffs (Darkfire! Divine power!) and weapon specialization, tons of healing, maximized blade walls, high reflexes plus evasion and so on and so forth. But that was more an over-powered combination of things than one individual over-powered spell.

 

[Omnidum]

I got a new one: Dices that only roll twenty in combat.

 

[Ace of Spades]

Evil Pentagram Soul Suck Horror (Infernal Wrath) from Fable: The Lost Chapters.

 

[SniperInTheSky]

Weapon/s:
-Sniper Rifle/ Scorpion Tank... "Yeah shoot at the tank ass-holes".1-2-3... HEAD-SHOT and/or Ka-Boom...

-The Cross Bow form BioShock. You Can shove the Grenade Launcher and Rapid Fire Shotgun up your sphincter... Give me Bolt-Trap with reusable ammo.

-Nightmare's sword... Even the squid is death incarnate in the right hands...

-Disgaea Mages... nothing is more heart warming than watching your cultivated mage totally destroy characters 10 to 1000 times their level... when their at level one.

Skills

-Chameleon... In Oblivion it makes you a god, in halo in makes you a dick, in BioShock it makes you feel like a "Predator", and in final fantasy 1 and tw0 in can be a friggin' life saver.

-Hacking, when ever this skill pops up in a game it usually means, "machines/electronics you are my *****"... then you give it a swift taste of your pimp hand... then go about your busyness.

Items

-Ghoul Mask... it take about a two thirds of the pressure of exploring in Fallout 3... period.

-Lockpicks... what more needs to be said?

-Frictionless upgrade from Mass Effect... Find your self a decent rifle put one High Explosive upgrade on it, fill every other empty slots with the frictionless, and nothing will be able to stand in your way...

 

[SniperInTheSky]

Oh I forgot... Pyramid Head's sword, if you could handle it you... you could kill almost anything in one hit form that point on.

Also, the Ripper and Cutter from Dead Space... Mostly the Ripper, it just rapes everything... except bosses, thats where Cutter comes in. And if they are both fully upgraded, you will no longer have any fear of what is behind any door or lies in wait in that "spooky hallway"...

 

[NeedAUserName]

The Spartan Lazer in Halo 3 and LMG's in COD 4. The lazer kills pretty much everything in one it and if you've played COD4 online you know what I'm talking about.

1, Spartan Lasers in my opinion are quite bad as I can never hit anything, but that could just be me.
2, LMGs, aren't that good, I can normally kill them with either an SMG, or a shotgun.

 

[Scary_Bob]

Concrete Donkey!!!

Also the Soul Reaver, doesn't do a great deal of damage but invulnerability + time + pointy thing = dead bad doods!

 

[OverlordSteve]

Avada Kadavra. Can't get more overpowered than that.

 

[Sennz0r]

The Omega beam from Metroid Prime: Hunters. I loved and loathed the thing. Everyone would always go to grab it, fire it and kill either everyone including themselves, or no one.

I'm greatful the damn thing was only available in one map.

 

[dekkarax]

Armageddon from the worms game with the same subtitle. It killed everyone.

 

[sirdanrhodes]

The RPG in HL2M.

 

[PureChaos]

The Iris summon in Golden Sun: Lost Age. it was difficult to get and has a high summoning cost but not only would it kill pretty much anything it would also heal you entire party including anyone that has died.

 

[Unknower]

AWP is probably the worst of the worst of overpowered weapons in FPSs. Let's see, it doesn't matter do you stand or crouch, no sniper sway, one hit kill, has enough ammo to kill most of the enemies with one clip and the accuracy is just completely ridiculous. Hell, you probably could defend most of the CS maps with 2-3 AWPs and the rest of the team could just spray logos and giggle at them.

 

[Khadath]

This ones for all the warhammer peoples out there.

-The Planet Killers(16km long space ship) Armageddon gun.

Than for FPs's.

-The Hammer of Dawn.

And finaly spells.

-Time control from Fable 2.

 

[OneHP]

Hero Fear - Lineage 2
Makes groups of enemies run away from you, either off a cliff or into a wall ready to be finished by friendlies while they can't do a thing.

 

[Fatalis67]

Sniper Rifles:
1-2 Shot kills
Pinpoint accurate
Shoots across the map with no accuracy loss
"It takes lots of skill!" Bullshit, I use it when I'm behind and need easy kills.