Probably another human being. We are so smart that we can defend ourselves against all other animals (and plants, because they can be predators too). Viruses and bacteria aren’t really predators, although they are more likely to kill us all.
But another person is as smart as us (on average) and they might want what we have, so I would say they are a type of predator.
If you are talking just about other animals, then probably a lion. Once they have tasted human flesh and how easy it is to get to us, then they can be very patient and will stalk and hunt us down. I’ve seen lions climb trees like cats, wait patiently for hours on end, and then attack almost silently.
Nothing predates humans. I.e. nothing hunts humans as their prey. Animals will only attack us if provoked, by entering their territory or threatening their offspring, or if there is no other prey around – they’re desperate (polar bears for example).
There’s one exception – humans. We are our only predators.
I think the biggest danger to humans comes from viruses and bacteria. They are incredibly fast to adapt and can spread through human populations really quickly. Most viruses and bacteria are mostly harmless, or make us feel ill for a few days but some can be deadly. Bacteria can become resistant to drugs very quickly, which means we need to keep creating new treatments all the time to keep up with the bacteria.