I would put Jacques Cousteau and Felix Rodríguez de la Fuente in the same league. If we take in mind the risks that they suffered to film nature I would say that they score higher in the epic factor.

I'm not denying that Attenborough is a category in himself and a wonderful narrator of course.

Freeman and Sigourney are in a different category. Both are excellent actors and narrators in films directed by another people. Freeman is "the" voice in the anglosphere, but Rodriguez de la Fuente was "the" voice in the latinosphere. Everybody was trying to imitate their style and accent decades after their death. He was the leader in a wolf pack when nobody was doing that, and don't hesitate to escalate a clift to take a good shot of a vulture nest. He was not an actor that just arrives to a set, say their lines and go.

Some people's work is of a sort that it doesn't get Americanized so much as alter American culture. I'd put both Adams and Cousteau in that class. How could Cousteau be understood through that accent? How could anyone not pay close attention once the accent was decrypted?

By the end of his life, Jacques Cousteau seemed a caricature of himself. The red cap, the thick accent--the Cousteau aesthetic was so overripe that director Wes Anderson used it as the template for The Life Aquatic with Steve Zissou.

https://www.outsideonline.com/outdoor-adventure/water-activi...

Morgan Freeman as a runner up

Ken Burns is a candidate, though not strictly natural world stuff.