I have seen so many silly top 10s on the Internet regarding this topic, including one made with AI, and some of them are absolutely ridiculous, putting even PSG or City over teams like Milan and Inter for example.
There are nine clubs that are sacred for the sport and should not ever be left out of any historic top 10, regardless of the order in which you put them and those are Real Madrid, Barcelona, Bayern Munich, Liverpool, AC Milan, Manchester United, Juventus, Inter Milan and Ajax. And no other team in Europe(and frankly, the world), is bigger than any of them.
After those 9, put whatever club you want, put a Portuguese one, or Arsenal, or Chelsea, or whatever. But those 9 are non negotiable and leaving them out honestly makes any top 10 look either ignorant or made by a really young person.
Edit: And I mean big as in overall trophies, status, prestige, players, ballon d ors, history, fans, etc. Not just followers on social media and revenue.
Real Madrid, Barcelona, Bayern Munich, Liverpool, Manchester United, AC Milan and Juventus are all locked in that’s obvious which leaves spots for two more.
I’d certainly say Ajax should be in the Top 9 given their three-peat in the Champions League. Inter Milan are certainly smaller than AC Milan and Juventus but they’re still comfortably the 3rd biggest team in Italy. Benfica and Porto are also a very good shouts given both clubs have won the European Cup twice along with massive success domestically in Portugal
Arsenal are a big club but no history in Europe unlike every other club mentioned combined with no little success in nearly two decades makes it very difficult to justify their inclusion in a Top 9 list. Same goes for Atletico Madrid as well. Dortmund have European silverware and as much as I personally like them as a football club, I don’t exactly consider them European royalty compared to the other teams either
Man City, Chelsea, PSG etc weren’t considered relevant until very recently and shouldn’t even be considered on this list