Rumble by a considerable margin.
Ngannou certainly has big power and looks like the future of the HW division, but I think there's a bit of recency bias going on for the people picking him. Ngannou has KO'd a bunch of low-level HWs, with the exception of Overeem; and Overeem has been KO'd stiff about 10 times.
Meanwhile, Rumble has KO'd a lineup of top 10 LHWs. He's brutalized guys who rarely get brutalized. He's a proven commodity. Outside of DC (and perhaps Volkan), every LHW would go into a fight with Rumble expecting to get KO'd. In Ngannou's case, he's far less proven. There are a number of HWs who would conceivably have a good shot at beating him.