Obligatory
not an oppo fan caveat, and I'd definitely rank United in the Top 3 overall!
1. Real Madrid: Always have been the #1, despite what other clubs claim, and will likely retain that spot for the foreseeable future — that's what I truly believe, even as a United supporter. While they don't always top the money league they're consistently there or thereabouts, they have a massive fanbase across the world — particularly in Spanish speaking countries or rather South America in general (which is the spiritual home of football with Europe), and their history puts them in another dimension: 13 European Cups and they've always been miles ahead of the chasing pack aside from the mid 2000s where it looked like Milan might catch up with them, 33 domestic league titles, 19 Copa del Rey — with 4 Top 10 players of all time playing for them at one point or the other — Puskás and Di Stéfano back in the day, Ronaldo for the Galácticos, and recently Cristiano — and that's aside from Figo and Zidane and Gento and Carlos and Laudrup and so forth.
2. Barcelona (by a whisker): A decade ago, they were on the same level as or even a bit behind Bayern and United and and Milan and Juventus, but the era of Messi and Xaviesta has propelled them to the second spot (which shows how rapidly the landscape can change even in the elite tier following special, generation-defining turn of events) — they've grown massively in terms of their fanbase (especially in the younger demographic — which leads to more sustainable growth), added 9 league titles to their haul as well as 4 European Cups, have the biggest stadium in European club football, captured the imagination of most football aficionados in recent years with what was arguably the most beautiful and best club team of all time, and a great history wrt. players: ranging for Cruyff and Neeskens to Romário and Laudrup and Stoichkov and Ronaldo and Rivaldo and Messi, and so forth.
3. Manchester United (decent lead over the rest of the best): I think what sets United apart from comparable clubs like Bayern or Milan or Juventus and Liverpool (who're all great on paper) is that it's always had a unique and compelling romantic charm — and has been loved across the world since the Busby era, even in periods where we weren't successful — whereas if you look at Milan now for instance, their legacy has eroded rapidly and their following/finances/gravitas have diminished by a significant degree just a decade after they were dominant in the Champions League. United has always had a massive following because of its story and been more erosion-proof — drawing people to packed stadiums or their television screens, magnetic players like Best and Beckham — who made United extremely “famous” — more so than the chasing pack, maximized their revenue with the Premier League era and growing popularity of football in English speaking developing countries (consistently towards the top and
#1 right now) on top of benefiting from the halo effect of the Ferguson era — which lingers to this day in a myriad ways.
Then the 4 giants, Bayern (suffer a bit because of the league they play in and traditionally didn't have a massive following in the international markets despite their success), Juventus (they've always been a bit dour and less fashionable outside of Italy than Milan though Ronaldo might change that especially considering their recent success relative to Milan), Liverpool and Milan (historic European powerhouses with great legacies and massive international followings) in a broad sense — one club might pull ahead at times, but they are on a comparable plane if you brush with a broad stroke, though Milan are in danger of falling a bit off the pace despite their illustrious history. And then Internazionale and Ajax.
As for the here and now, United is definitely the gold standard for money and following — but unfortunately, not in terms of on-the-field organisation or decision-making or player personnel or recent success if you consider the post-Fergie era in isolation (it's important to make that distinction because including his time dilutes a more grounded present day assessment). There are 4 cleat-cut elite clubs right now: Madrid, Barcelona, Juventus and Bayern — probably in that order — gargantuan institutions and successful on the pitch. United can claim a spot in that that bracket if we improve rapidly because the institution itself has all the tools and massive appeal, so the onus is on the decision-makers and the manager(s) and the players, but in a purely footballing sense I'm afraid the two petro-clubs have pulled ahead in the short term, and Atlético can definitely stake a claim even though it's a much smaller institution. Too early to tell with Liverpool — definitely on the rise, but I'll withhold judgement for a bit longer.