Hard to say. The Old Gods play more toward manipulating and subverting other races to do their bidding and their power structure is severely weakened, despite the threats associated with them. I'd say Sargeras and the Burning Legion are at least a much more immediate threat. No one has really battled Sargeras and when you consider how little the Burning Legion has interacted with the races that inhabit Azeroth (in comparison to the Old Gods, anyway) I would say the Burning Legion are a much larger threat (they've destroyed entire worlds that we know about).
When you also consider how much damage the Burning Legion is able to dish out or responsible for in relatively short periods of time (such as with Hyjal or the Lich King), how badly everyone was freaking out when Kil'jaeden almost entered Azeroth and how terrified Wrathion seems of the knowledge of an impending Burning Legion invasion, I'm inclined to say Sargeras.