Tier 1: Real Madrid followed by (with a distance) AC Milan
Tier 2: Liverpool & Bayern
Tier 3: Barcelona
Tier 4: United, Inter, Ajax, Juventus
Those who put Barca above AC Milan, Liverpool, Bayern probably think football was invented early 2000s.
Juve could make it to Tier 2 if they didn't lose that many UCL finals, United too if they could win couple more..
Your post is basically the answer to the question: who has the most CL titles?
But it's a lot more complicated than that.
How can AC Milan be the 2nd biggest club in the world, when they can't even win Serie A these days, which is not even a top 2 domestic league anymore?
-3rd most Scudettos with 19 after Inter wins their 20th title in a few weeks
-Won 4 of those 19 titles in the last 25 years
-Last won the Champions League 17 years ago
-Can't even compete with most PL clubs in the transfer market, let alone the big 7
-Their highest paid player is Rafael Leao with 147,000 per week (they have 2 players above 100k, check PL clubs for comparison)
AC Milan's only hope of returning to the status of a top 3-5 biggest club in the world in the current climate is the Super League.