As you already know TGG has begun his ladder of epic duels of epicness. Naturally, I ganked the idea and am doing the same thing. My combat structure consists of two separate ladders: one for fictional characters, and one for Futuristic galactic governments.

We did our best to remove the unkillable murder machines that developers love to use as plot devices. We also tried to keep people in fights with access to similar technology. I don't care how many ghosts Aragorn had at his back the Chief with a Spartan Laser would still win.

Governments:

Terran Dominion
I've seen that in: Starcraft
Why they can win it all: Nuclear Arsenals

Cylons
I've seen that in: Battlestar Galactica
Why they can win it all: Robot Overlords

Yor Collective
I've seen that in: Galactic Civilizations
Why they can win it all: Death rays

Covenant
I've seen that in: Halo
Why they can win it all: Plasma

Advent
I've seen that in: Sins of a Solar Empire
Why they can win it all: Psionics

Galactic Empire
I've seen that in: Star Wars
Why they can win it all: Speed and Lethal Efficiency

The Citadel Council
I've seen that in: Mass Effect
Why they can win it all: The ability to accelerate projectiles to FTL speeds

Galactic Federation
I've seen that in: Star Trek
Why they can win it all: Super Torpedoes

Fictional Characters:

Rorschach
I know them from: Watchmen
Rorschach is the obvious king of bad-assery. He may be insanely paranoid but that doesn't stop him from kicking some major ass.

Walker
I know them from: Walker, Texas Ranger
Chuck Norris can fight bullets with his fists.

The Joker
I know them from: Batman
The man without a plan is here to cause some pain. He matches Rorschach's insanity but is crazy in a very different way.

Il Duce
I know them from: The Boondock Saints
Killing in the name of a greater good usually fails. That is unless you can constantly bring the heat. I'll be the first to officially nick-name Il Duce as "The Oven".

James Bond
I know them from: DUH, James Bond Movies
The classiest killer on the list. Don't mess with the British Badass.

Jules Winnfield
I know them from: Pulp Fiction
See Ezekiel 25:17
 
Antonio Montana
I know them from: Scarface
He jacked on crack and ready to fight some cock-a-roaches.

John McClane
I know them from: Die Hard
I refuse to use the phrase on the grounds that I've heard it enough.

Rambo
I know them from: Take a guess
Did I mention that he's fighting without the shirt? He seems to do better without it constricting him.

Captain Price
I know them from: Call of Duty: 4 Modern Warfare
The man does whatever he wants to, and does it with a cigar.

Buddy Israel
I know them from: Smokin' Aces
Card tricks aren't just for magicians anymore.

Solid Snake
I know them from: Metal Gear Solid
Did that box just move?
It did, and it doesn't want you around anymore.

Mattias Nilsson
I know them from: Mercenaries
He's a lean mean Swedish death machine.

Sam Fisher
I know them from: Splinter Cell
Where'd he go?

Gordon Freeman
I know them from: Half Life
This is him without the Gravity Gun or any Combine technology. He is still a strong contendor, even though I might leave him with only a crowbar.

V
I know them from: V for Vendetta
Shoot away, but the knives will get you.