I will say Tyranids both in the long and short run. Necron Harvests are truly devastating but rarely does a Necron threat extend beyond a single planet or system or beyond small fleet actions, very rarely. But to me it seems the Tyrandis very rarely are contained to a single system. Every encounter with the Tyranids resulted in hundreds or thousands of systems and dozens of sectors either being mobilized, devastated, consumed, or cordoned off and isolated due to a Hive Fleet marching in. Tactically Necrons are a bigger threat but in the grander scheme of things Tyranids represent a larger strategic threat.