Ok, after much research, thought & deliberation I am going to try to provide some kind of answer to this debate. Bear with me, here goes.
To define greatness is the hardest part of the arguement. Obviously every Juventino will say il bianconeri are the greatest club in the world, otherwise how could that man claim to be a Juventino, why support them if you don't hold that very belief close to your heart?
However, trying to be unbiased & objective about this, which club(s) can claim to be greater than Juventus? Firstly, who are the greatest teams from each major footballing nation? I think the definative list would read:
UK - Liverpool
Germany - Bayern
Netherlands - Ajax
Portugal - Benfica
Spain - Real Madrid
France - Marseille
Italy - Juventus
These clubs are chosen on domestic sucess, fan base & world wide support/identity.
Now, how to rank their domestic achievements? I figured the best ranking system is that used by UEFA in the Golden Boot top scorer rankings, which gives each league a multiplier, depending on strength of the league. Spain, Italy, Germany, and England are all given a multiplier of 2, Dutch, French & Portuguese leagues a 1.5. So, mulitplying the number of titles won by this number gives the following results;
Real - 58
Juventus - 54
Benfica - 46.5
Ajax - 43.5
Bayern - 40
Liverpool - 36
Marseille - 16
Then I felt the need to add domestic cups, at the value of half a league title, and UEFA cup victories at the same value as a league title. Factoring in CL/European Cups was more difficult, but finally decided that it must be worth 2 league titles. At this stage I also felt the need to bring Milan into the equation, as their continental sucess far exceeds our own, as well as Manchester Utd & Barca, due to their worldwide reputations. The following points totals were thus reached;
Real Madrid - 97
Juventus - 77
Bayern - 71
Liverpool - 69
Ajax - 69
Barca - 68
Benfica - 67.5
Milan - 51
Man Utd - 51
Marsielle - 30
These rankings show that the clubs I added rank some way behind the original list, & that Marsielle are even further out & thus are discounted from the discussion. Looking at the above table I find the top 5 to be a fair reflection of the rankings in Europe in terms of size, sucess & percieved "greatness". Real Madrid ARE head & shoulders above the rest, esp when you consider that in 51 years of the European Cup, they have won approx 1 in 5. Our domestic glory, even only counted 27 titles (that nearly killed me!), added to our huge worldwide fan base, plus 2 European cup wins & 5 losing finals apperances is enough to put us above Bayern, who's league is somewhat weaker, despite 4 EC wins. Liverpool & Ajax make up the top 5, & rightly so. Fans of all 5 clubs could argue all day about the final order they should take, but clearly these ARE Europes top 5 clubs, & I feel my ranking system is fair.