President Grant and his wife.
All my life I thought that was the trick answer to the question. But it turns out the bodies are above ground in the tomb, so nobody's buried in Grant's Tomb.
They say Groucho Marx made the question famous on his TV quiz show. According to Wikipedia, he accepted pretty much any answer (nobody, Grant, Grant and Mrs. Grant) just to have an excuse to give some money to people who couldn't win it otherwise.