Do you know what the phrase, "OF ALL TIME" means? If you think it has something to do with time then you're right!
It means the best throughout the ages, of all possible time. So if we say the movie Pretty Woman is the most romantic movie of all time, it means it's the most romantic in this lifetime, no other movie could ever compete with it and win this title.
It might help if you try to substitute the phrase "OF ALL TIME" with the word "EVER"
He's on of the greatest actors of all time (EVER)
Whitney Houston had the greatest voice of all time (EVER)
What's the most romantic movie of all time (EVER) ?
Can you make up your own sentence with this phrase?