What does it say in the one ring of power?

Answer 1

"One Ring to rule them all, One Ring to find them, One Ring to bring them all, and in the darkness bind them"

Written in Black Speech using elven script, which is depicted at the link listed below.

It is an excerpt of a poem about the ring which goes like this:

"Three Rings for the Elven-kings under the sky,
Seven for the Dwarf-lords in their halls of stone,
Nine for Mortal Men doomed to die,
One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie.

One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie."

Answer 2

One ring to rule them all.