I would give it to the Legion. The Old Gods were stronger in the War of the Ancients trilogy only because the Legion was invading via a portal. If they ever find Azeroth, the planet will quickly be destroyed, with the Old Gods.
The legion only seems powerful because they have millions of demons to just throw at people. They of course have their more powerful demons, but from what we have seen even their power wouldn't come close to any Titan or Old God.
The Old Gods don't seem like beings who could easily tire and would be capable of mass destruction.
So I would imagine their only real trouble would stem from Sargeras and even then 3 or so Old Gods fighting him they could probably destroy him.
Then again we don't know much about the Legion or Old Gods power.
If the Old Gods could completely ignore armies, then they'd have no need for armies, themselves.
Except they do.
When the Pantheon came to Azeroth, the bulk of the fighting was done by the Old God armies - the ones they previously used for their own power struggles. The ones led by their Elemental Lords. Once that war was lost, the Old Gods had their citadels smashed and were chained.
The Old Gods we've seen since have very much followed the same mode of operation.
The Old Gods do not follow the Hero style of leadership. The Old God is not the one leading the charge. The Old God stays in the back, where it's safe, and sends forth minions to do its fighting for it.
They do not personally wad on ahead and annihilate armies. They only end up facing an enemy personally if their plan has broken down - and they don't tend to fair too well.
... Really, despite their "team up on Sargeras" plan from back in the War of the Ancients books, I'm not even sure it'd be physically possible
. The Old Gods are not very mobile beings. That book was written long, long before some of the more modern developments related to the Old Gods happened.
If the Old Gods lost in the field against the Legion (Old God armies vs. Legion armies), Sargeras might not even need to personally visit each Old God's prison/hiding place, one by one, and impale it with his super-weapon. He might just send in legions of demons armed with Elementium (a metal that hurts Old Gods) to chop them up.