Eldrad already wrecked Failbaddon, and being the only real psyker gives him a very high advantage over the others. In a straight fight he'd probably lose, using even a little strategy he'd win.
That's a good question, if we're talking tabletop, I'd give it to Ghazzy if he's Waaaghing that turn. If we are talking pure setting and story... my money would still be on Ghazgkull.
It's simple, the Swarmlord. He is virtually unkillable, as he'll just be reincarnated into another hive fleet, and he has almost killed Calgar already. His swords will make a mockery of any armor, his parry is overpowered, invulnerable saves are next to useless, he's 20 feet tall, psycic powers pretty much can't hurt him, and he's a psyker himself with the full power of the hive mind with him.
