Most powerful being.

[Pater Sin]

I think Vect or Lelith would give as good as most of the other characters, plus nobody has mentioned any DE yet.

[AM-254]

On the table top (i believe) Draigo is now just AP3, where the armless wonder has been FAQ'd so that his sword is AP2. So assuming an actual fight between the two were to take place Draigo, slayer of Daemons and menacer of the Ruinous powers would probably manage to, at best, give Abbadon a few scratches before being stabbed throught the arse with Drach'nyen, storm shield or no storm shield.

Also, fluffwise Prince Yriel of Lyanden wields a spear containing the power of a star and uses it to KO a tyranid beast so powerful that it "could not be slain by mortal weapons". Other characters which it is generally agreed cannot be slain by mortal weapons include daemon primarchs, greater daemons and other such nasties. Presumabley a weapon this powerful would, in the fluff, allow Yriel to turn the galaxies greatest heroes and villans into the wierdest kebab ever seen in fortyfive thousand years of human civilisation.
Ofcourse, on the table he can be curbstomped by an imperial guard officer with a powerfist :P but that's Eldar for you

[Power Klawz]

I'm going to second the vote that its whatever hapless servitor happens to be assigned to dusting the ancient surge protector that the Golden Throne plugs in to.

[White Tiger88]

I'm going to second the vote that its whatever hapless servitor happens to be assigned to dusting the ancient surge protector that the Golden Throne plugs in to.

I third this.....ouch......

[droozy]

I think a Bloodthirster would run through most of these choices. If not a playable character then the emperor(even as a corpse) and the chaos gods. Everything else is just over-thinking it.

[CraftWorld]

A single playable character? It depends on circumstance I suppose but for me it's hard to think beyond Draigo.

As for fluff, Eldrad?

[neutrino]

how about skarbrand, in the new daemons codex, dont think u can find a better one v one fighter in the game

[phreakachu]

Im suprised that no one has mentioned Lucius yet. the dude will not die.

[Crazy Jedi]

No mention of Sly "Drinking Buddies with Chuck Motherf*ing Norris" Marbo?

But I think serious fluff wise it would have to be Abbadon, incredibly strong, master of a massive army, and an incredible tactician (depending on how we view his 13 Black Crusades)

On the tabletop, I've seen Abbadon die three times, I've only seen Draigo once, and he seriously messed up a 15man unit of Black Templars with a kitted out Chaplain and Marshal.

[Psychosplodge]

Chuck Norris? pfft Chuck Norris is Christopher Lee's B!tch.