It's Ajax by a mile.
Measuring how "big" clubs are is dumb, but if you're going to do it, it's Ajax.
They've put a much bigger stamp on the history of football. Rinus Michaels, Cruyff, Total Football - Barcelona's entire identity is built on copying Ajax, their academy based on Ajax's, their best teams built either by Cruyff or his greatst disciple (Pep). In turn, our entire academy, the style of play our upper management is trying to create is based off Ajax's via Barcelona, and we're not the only ones either.
It's not just that they've won the European Cup 4 times, the teams that did it were reference points for decades later. The "Gloria Ajax" years and total football, then the van Gaal youngsters with Kanu and Overmars and Seedorf.
Not to mention the talent factory they are - van Basten, De Boer, Rijkaard, Bergkamp, Davids, Seedorf, Cruyff... you could literally name 50. Not all in the past either, they still constantly pop up at the top of the lists of clubs with most academy products in the top 5 leagues in Europe.
6 Ballons D'Or out of that club, none for Arsenal.
Then there's the usual stuff - the only club in one of the world's greatest cities, the biggest club by a mile in one of the world's great footballing nations.
Arsenal could stop existing tomorrow and no one would really notice - Spurs and Chelsea and West Ham and Fulham and Palace would just swallow up their fans.
So trying to work out what clubs are bigger is stupid, but in this case it's Ajax.