Are killer whales at the top of a food chain?

Yes. Usually nothing attacks a killer whale (although a shark might go after an unprotected calf), and once in a great while, humans will hunt them for food and/or oil. Killer whales are not the biggest toothed mammal, that is the sperm whale. Sperm whales eat fish and squid, not killer whales. Killer whales almost never attack the sperm whale.