Not accounting Eldrad is the most illustrative of humans and other aliens. What do you know about destiny and manipulation?
I'm not a huge fan of the Ultramarines, nor am I the biggest fan of Matt Ward's work, but judging by what has been written about each of these characters, my money is on Calgar.
I think Eldrad would have a better chance if it was all of them rather than a one on one since Ghazhgull would probably rather fight the two large human rather than the small eldar, Calgar and Abaddon would probably focus on each other or Ghazhgull, so Eldrad might be able to wait out the fight until the other combatants are weakened or dead and then finish off the last one while he is tired from the fight.
My money is on Abbadon. FRAK THE FLOW, I AIN'T GOING MAINSTREAM. But in all seriousness, Abbadon is a bit under-estimated IMO because of all the "armless failure" jokes. That is pretty much the reason why I am putting my money on him. Besides, if Abbadon was such a failure, he should have been replaced by now... either A) The Dark Gods have no one else, or B) Abbadon is more powerful then anyone really realised...
Orkses is never defeated in battle. If we win we win, if we die we die fighting so it don't count. If we runs for it we don't die neither, cos we can come back for annuver go, see!