Who IS the Lord of the Rings?

LOTR -- Book II -- Many Meetings

'Hurray!' cried Pippin, springing up. 'Here is out noble cousin! Make way for Frodo, Lord of the Ring!'
'Hush!' said Gandalf... '... The Lord of the Ring is not Frodo, but the master of the Dark Tower of Mordor...'

I am not the most schooled in Middle Earth, but I am fairly certain the "master of the Dark Tower of Mordor" is Sauron.