The pair have been performing at the pinnacle of their profession for the past two decades and have the greatest respect for one another.
Indeed, Casillas recently reacted to a video of the duo in action by taking to Twitter to ask his colleague, "What do you think? For me, you're the best."
Buffon responded, "I don't choose. We are both the best!"
However, Goal has now decided to ask you, our readers, whether you think either man deserves to be considered the finest goalkeeper of all time.
There are obviously several outstanding candidates. For some, Soviet Union icon Lev Yashin remains the greatest of all time, while others believe that Dino Zoff's longevity cannot be overlooked, given the Italian led his country to World Cup glory at the age of 40.
Of course, Gordon Banks will forever be associated with the game's grandest stage thanks to his sensational save from Pele in Mexico in 1970 and let's not forget Thomas N'Kono, one of the stars of Cameroon's sensational run to the quarter-finals in Italy 20 years later.
Modern great Manuel Neuer, who won the World Cup with Germany in 2014, is credited with mastering the role of the 'sweeper-keeper' but Jose Luis Chilavert used to venture even further forward, to take free-kicks and penalties for Paraguay!
So, who's your No.1 No.1?! Cast your vote now!