S tier, probably in this order:
1. Real Madrid
2. Barcelona
3. Manchester United
Then, A tier, in no particular order: Liverpool, Bayern Munich, AC Milan, Juventus
And then the rest.
However, there are certain aspects of being a big club IMO. You need to have all of these in order to be able to break into S tier: Money, Big stadium, Huge following globally, Huge "pull", and a time period of sustained success in the modern era
1. Let's look at Liverpool, for example: A true historical giant if you look at trophies won, they have a big stadium, a huge global following, however, do they have as big of a pull as Manchester United? No. Do they even have a bigger pull, than Chelsea or Arsenal? I don't think so. They don't even have the financial strength of these 3 I just listed.
Why? I think because they won most of their major honours before football became such a big global phenomenon in the last ~30 years. That's why United have a bigger following, a bigger stadium, more money, and a bigger pull. Because of what we achieved between 1992 and 2013. Sir Alex Ferguson's era, 13 Premier League titles, 2 Champions League trophies, a lot of FA Cup and League Cups, a 75 thousand capacity stadium, countless legendary players, countless unforgettable moments and memories, and very importantly, the fact that the Premier League became what it is today. If it wasn't the biggest, best, and by far the richest league in the world, we could've easily lost our status as a top 3 biggest club in the world, just like AC Milan sadly did.
Why are Barcelona in that same category? Because of their La Masia golden generation with Messi, Xavi, Iniesta, etc. and what they achieved from the mid-to-late 2000s until around 2015. They would still be a big club if that never happened, as they had multiple Ballon d'Or winners play for them before those ultra successful years as well, and they were a CL winning club before too, and every now and then they won a few La Liga titles in the 2000s, 90s and 80s as well. The rivalry with Real Madrid also helps, as does being located in sunny Spain...but they wouldn't be what they are today without those 9 years from 2006 to 2015.
So, overall, Madrid and Barcelona will likely be the top 2 for decades to come, unless something goes really bad for one of them. They'll always be able to attract the best players in the world, Barcelona will always produce world class talent, both teams will have all-timer players play for them in the next decades, as it has been the case since forever, basically.
However, I feel like we are the best suited club to rival their greatness. Even in these last 11 years, we've maintained everything I bolded in my first paragraph, except success on the pitch. Imagine when that comes.
