The Scourge is known as one of the strongest empire in Warcraft Lore. They can raise and turn their fallen enemies into more scourge troops. They do not need food nor do they get tired. They cannot be corrupted by anything, not even the Old Gods. Even if someone is immune to their plagues. They would still be able to reanimate that person once he or she dies. And even if something can't be resurrected into a undead. The Scourge can still use their fresh to make abominations. In order for a empire to defeat the Scourge empire. They would need a few key traits. The empire must be large and powerful. They'll need to be aggressive and gain a huge amount of momentum. Being immune to the plague or immune to becoming undead would help greatly. Luck would also be a good factor here.
Edit - instead of only empires, let's count any form of factions so for example the Genesaur, Magnarons from WoD, Mumur etc counts too.
Below are the factions I see that could defeat or at least have a chance of defeating the scourge.
The Burning Legion (not counting 7.3 - They have stronger pound for pound troops in almost all units plus they are far larger. The Legion had to fight every major faction and hero at the same time which is the reason they seem weaker than they should in Legion. Still in the warlock destruction spec quest. The Legion would had gotten a huge amount of reinforcements if they haven't been stopped. I believe the Legion could defeat almost any faction if they were to focus on one at a time.
Black Empire - The Black Empire at it's prime has the Old Gods, Aqir, and Faceless Ones. This empire should be able to steamroll the Scourge easily.
Aqir Empire - The united insect civilization would stand a good chance against the Scourge. They are a powerful remnants of the Black Empire.
Zandalari Empire - the Prime Zandalar Empire even managed to defeat the Aqir Empire. Might stand a decent chance.
Kaldorei Empire - The ancient Night Elf Empire is large enough with extremely powerful magic. They should have a chance too.
Ethereal Empire - At their primes, they seem to have a whole world under their control before it got destroyed by a Void Lord.
These are the ones I think could defeat the Scourge or at least stands a chance against them. The ancient Mogu were powerful but the tweet already confirmed that the scourge would defeat them. The others like Illidan's army, Deathwing's army, Iron Horde etc would all die easily.