By far and away the biggest enemy of sharks are humans. Humans kill 73 million sharks every year, mostly for shark fin soup, a delicacy to the Chinese diaspora.
In the wild, a few other animals can kill sharks, like a large crocodile. Dolphins in groups have been known to chase away sharks,, but aren't really enemies. They only predator that could probably regularly take on a shark are Orcas (killer whales) that have been seen attacking Great White sharks, but sharks don't appear to be regular prey.