In the pantheon of great movie stars, Marilyn Monroe might well be considered the brightest of them all. The actual Pantheon was a temple built by the Romans — but we use it to mean any group of exalted figures.
To place someone in a pantheon is to decide that they belong to a "club" whose members are all only the greatest people to have done the same thing. It's like a little temple of little gods. Tiger Woods, for example, definitely belongs in the pantheon of great golfers –– alongside Palmer, Nicklaus and Singh. If you want to see the real Pantheon, that is the building itself with the world's oldest concrete dome, then you'll have to go to Rome.