Old Gods. Without a shadow of a doubt. Sargeras can be killed,
Old Gods cannot be killed, not because they are immortal, but because they are directly tied to the very stability of Azeroth itself. To destroy the Old Gods would mean the destruction of the very planet. Sargeras had neither the strength, nor the foresight, like the rest of The Pantheon, to stop the Old Gods from doing this when they first visited Azeroth. Here's some nice info dealing with the most powerful of them all, N'zoth.
When Malfurion finally awoke from the Emerald Dream with the help from Broll and Thura, it is remarked, "When Malfurion Stormrage contained the Nightmare within the Rift of Aln, he sensed an "ancient evil" that was keeping its foothold in the Emerald Dream from somewhere beneath Azeroth's oceans."
Source:
http://wowpedia.org/N%27Zoth
From Krasus: "Krasus speculated that should the Old Gods open the gates of their prison, even Sargeras would find himself pleading for the peace of death."
Also about Xavius: "Although the Nightmare Lord in the dream turned out to be the Satyr Lord Xavius, it is suspected that he acted on behalf of a power even greater and darker than Sargeras."
Source:
http://wowpedia.org/Old_God
They are the complete embodiment of evil and corruption. Sargeras is a mortal, just like the Old Gods. He can die, be corrupted (which he was slowly corrupted over the countless millenia of fighting against demons. So even Sargeras had a breaking point to where he couldn't handle his duties anymore.), and be slain. He is powerful, but the Old Gods are unflinching in their desire to corrupt and dominate all life. Unlike Old Gods, Sargeras' death would not destroy all life and the planet, as Old Gods are directly linked to the world. The Titans found this out when fighting them. C'thun and Yogg-Saron are not "dead", but instead, their husks have been beaten, while their existence lives on. C'thun and Yogg-Saron are still fully able to be resurrected.
Also, it should be noted that all the various demonic races of the Legion are controlled by corruption and dominance. The Old Gods are the living embodiment of these forces, and are capable of overpowering even Sargeras' magic in doing so. If the Legion was to come to Azeroth in full-force, it takes only a whisper from an Old God to dominate all of them, therefore stripping Sargeras of his power. Sargeras is simply the main antagonist, because he is a tangible enemy that can be dealt with.