Messi has won the Ballon d'Or for the past three years but there are those that believe his Portuguese rival remains the more complete player.
Xavi disagrees, though, arguing that his Blaugrana team-mate is the far more influential of the two.
When asked during an interview with Gol TV if Messi was the best in the world, the Spain international replied: "Yes, and I think most people in football think as I do.
"Obviously Cristiano is a great footballer. They are two players who make the most difference on a football field, but Messi is above anyone.
"I hate to compare, but Leo gives us everything: scores goals; creates them; always involved in the flow of the game, leading nearly every attacking move.
"This guy has it all: he even heads the ball well in spite of his height. Pep [Guardiola, Barca coach] once said that he dominates every facet of the game, and he is absolutely right. He is the best, number one."
Turning his attention to the Clasico, Xavi insisted that the Catalans will approach the game like they do every other but admitted that they will probably play with even greater adventure than usual given they go into the game four points in arrears of the league-leading Blancos.
"We try not to change our philosophy; to always be protagonists," the World Cup winner mused. "Madrid have played in various ways but we have not changed out style of play in any game.
"We have always wanted the ball in order to have those long possessions that help us control the game and, from there, try to win the match.
"We cannot change our game now. We have done it against all rivals, and against Madrid at their home, too.
"So, we will try to attack more so than ever, having the ball more than ever and go on the attack more than ever to win the match."
Barcelona's eagerly-awaited clash with Madrid kicks off at Camp Nou at 20.00CET.
Goal.com will be hosting a LIVE Twitter Q&A on Friday at 14:00BST as Goal.com Spain Editor Martin del Palacio Langer and Goal.com International Features Editor Carlo Garganese debate whether Lionel Messi and Cristiano Ronaldo are the two greatest players of all time. Throughout the LIVE we want to hear your thoughts and opinions on the subject, while we will also be putting your questions to Martin and Carlo as we look to bring the debate to an end.