Given their astonishing talent, the remarkable goals that they score and their ability to produce on big occasions, you could make a strong argument either way.
Most go with Barcelona's breathtaking Messi, which frustrates the Real Madrid superstar who will be out to enhance his glowing reputation when, as Portugal skipper, he faces Russia in tomorrow's World Cup qualifier in Moscow before taking on Northern Ireland in Porto next Tuesday.
Ronaldo v Northern Ireland. How scary is that?
Ronaldo has faced our wee country before in 2005 when, as a young Manchester United player, he was jeered during a friendly at Windsor Park.
It finished 1-1 with Northern Ireland players and fans still on a high after beating England 1-0 in Belfast two months before.
The match in Porto will be a massive departure from seven years ago because while Northern Ireland have gone backwards since, Ronaldo has improved out of sight, so much so that he is rightly regarded as one of the greatest ever to play the beautiful game.
But is he the greatest to play against Northern Ireland?
Now, there's a question.
Legendary footballing figures Pele and Diego Maradona never took on our national team, so they are ruled out.
George Best, of course, played FOR us, not against, so he is not in the equation either.
And while Messi still has time, he has yet to experience the delights of facing the Green and White Army.
Many other players, at one time or another labelled the best in the world, have, however, opposed Northern Ireland.
Take a look through the team sheets and the names that hit you throw images of artistry and magic into your mind and recollections of stories of glorious feats in days gone by filter through the brain.
Down the years Northern Ireland teams have come up against so many fantastic footballers it is tougher than stopping Ronaldo or Messi to choose which one deserves to be named the best, especially when you are comparing players from different eras, but then isn't that part of the reason why we love the sport so much?
Debate, conjecture — football fans live for this kind of stuff as much as seeing their favourite team score a goal.
And never afraid to voice an opinion, the Belfast Telegraph Sportsdesk are attempting this monumental task.
Elsewhere in these pages, you will see our top 10 and 40 other outstanding stars who make up 50 greats who have faced Northern Ireland.
Current Manchester United pair Wayne Rooney and Robin van Persie miss out along with priceless performers from yesteryear like France's Patrick Vieira, Dutch hero Johnny Rep, Germany goalkeeper Sepp Maier and Scotland's Jimmy Johnstone.
From those included in the shortlist of 50, Paolo Maldini, Socrates, Bobby Charlton, Kenny Dalglish, Roy Keane and Xavi could have made the top 10.
Those who were chosen are amongst some of the most iconic figures ever to play the sport — men who have taken football to places we never knew existed and in many ways change the face of the beautiful game.
Ronaldo is included, but ahead of playing Northern Ireland next week, he does not earn top spot in this illustrious list.
Apart from Portugal's finest there are two Englishmen, two Spanish, one French, one Dutch, one Brazilian, a German and a great Dane in the top 10.
The two England players are Bobby Moore and Duncan Edwards, one who was able to fulfil his destiny and sadly one who wasn't.
The Spanish influence is provided by the past and present, the mercurial Alfredo Di Stefano and the mesmerising Andres Iniesta.
Zico and Michel Platini, who bent the ball way before and far better than David Beckham, ensure that Brazil and France are represented while the pride of Denmark, Peter Schmeichel, merits his place for being the best opposition goalkeeper ever seen at Windsor Park.
And that leaves the supreme Johan Cruyff and Franz Beckenbauer from those old rivals Holland and Germany.
Cruyff, who faced Northern Ireland in the 70s, and Beckenbauer who took us on in the 60s, are giants of the game, dripping with greatness that will forever stand the test of time.
They are one and two in our list. Beckenbauer was and is the best ever in his position. Crafty, cool, classy Cruyff simply had it all.
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