An'ggrath
An'ggrath the Unbound, the Guardian of the Throne of Skulls, Most Favoured of Khorne, Lord of Bloodthirsters and the Deathbringer is the most powerful of Khorne's greater daemons, and should by rights then be a hard ass bastard who would reduce neckbeards to manly tears and gnashing teeth of jealously at his bad-assness.
Instead, in an hilarious example of fail, he suffers from a severe case of Avatar of Khaine syndrome: almost each of the times he has been mentioned in fluff he has been soundly beaten and sent running back to his master with his tail between his legs. Perhaps he represents the concept of loss in war? No idea, but it would go a long way to explain why he sucks at winning.
During the Horus Heresy, Lorgar, weakest of the Primarchs, had to face An'ggrath, in a duel to prove his loyalty to the Chaos Gods. You'd expect Lorgar, who got kicked about by his brothers, to be well and truly curb stomped here but no, he gave An'ggrath a well resounding butt kicking instead. Did my cup of tea just fall from my hand in surprise? I think it bloody well did. Clearly the plot armour here is strong.
The first time he was summoned into the material plane was on the hive world of Ashur where Khornate cults were being purged by the Metamarines chapter. In desperation they invoked An'ggrath's name and summoned him, where he proceeded to BUTCHER EVERY LIVING THING ON THE PLANET. It took him 7 years to do so, which some would think is too long for such an angry winged tower of strength and savagery with an axe and whip, but consider that that he has to chop his way through entire hive cities' worth of Guardsmen and gangers, as well as an entire chapter of Space Marines. Add in that he has to fly from hive to hive, and it could very well take him those 7 years, after which he just disappears. Therefore he was not defeated and banished back into the warp, but simply ran out of things to chop apart.
The second time he was summoned was during the Siege of Vraks. Towards the end of the conflict An'ggrath finally decided to show his face, perhaps hoping to steal some of the glory while no-one was watching. Unfortunately for him the Inquisition got a whiff he was creeping around and one of their Inquisitors, armed with a potent toothpick, promptly pricked An'ggrath and sent him yelping back to Khorne's throne (although to be fair the toothpick was made specifically to kill ultra-daemons so that helped Inquisitor Hector Rex cut off An'ggraths knackers).
The Blood God did sigh and put his head in his hands, shaking it with despair. What is an evil god to do when the best he had is something that cannot even take out one, unaugmented HIGHLY AUGMENTED, unsupported Inquisitor?
Well, call Forge World of course! Thus they made a model (though good ol' GW has made a cooler one) and rules for him. To be honest he just looks like a normal Bloodthrister pumped full of steroids. Don't do drugs kids! You'll end up all red, veiny and unable to do anything but RAAAGGGEEEEEEE.
Canon is shifty on whether Ka'bandha or An'ggrath is in fact the most powerful Bloodthirster, as multiple conflicting sources and titles each state that one or the other is in fact more full of RIP AND TEAR. Matt Ward favoured the former, a clear sign you should favour the latter.
On the Tabletop
Finally, An'ggrath doesn't cost 888 points anymore, now sitting at a more comfortable 700, and he got some nice buffs too. Incredibly difficult to kill with T8, 24 wounds and a 2+/4++, not to mention that he can Deny two psychic powers and gets +3 to any Deny rolls he makes. He's a pure beatstick, with his only aura simply allowing Khorne units within 9" to use his leadership of 10. However, he is one hell of a murder machine, striking at S16 AP-4 on the charge with d6 damage and 11 attacks. His attacks, strength, and movement go down as he takes damage (although it's pretty hard to damage him in the first place). Any units he charges into combat with take d3 mortal wounds on a 2+. On a 4+ any enemy units within d6" when (how?) he dies take d6 mortal wounds. As icing on the cake, he has an assault 2d6 Bloodlash (plasma pistol profile with 2 damage) which can be fired into and out of combat. Literally nothing in the game can stand up to him for long.
Like all the uber-daemons, he can be summoned with an "arch-daemonic ritual", where the summoner rolls nine dice to try to reach his power level. Doubles inflict mortal wounds on the summoning character, triples kill them outright (but the ritual can still succeed).
