(the best/most/worst/least/etc.) of all time

If you want to compare something to absolutely everything else, use the phrase "of all time":

The '80s were the greatest decade of all time!

This means that there has never been a period of time better than the 1980s.

I'm the greatest singer of all time.

This means that no one has ever been a better singer than you, ever.

This phrase appears in these lessons: