Well a quick one would be the last great United side coming up against Peps Barca in multiple CL finals. There was competition. At the moment I feel as though there's a huge transitional period in European football with new managers coming in everywhere and old guard players being phased out. The only two teams hitting their peak are City and Liverpool.
08/09 United were a great side. The last truly great english side before City and Liverpool. That season, the truly great european sides were Barcelona, United, Chelsea and Liverpool
10/11 side was 18/19 spurs with a luckier CL draw
If you look at the great sides in Europe, year to year, you get Barcelona and Real Madrid 10/11, those two plus Bayern and Dortmund 11/12 and 12/13, 3 of those 4 plus Atletico 13/14, same 3 plus Juventus 14/15, same 4 plus Atletico 15/16, back to barca/madrid/juventus/bayern 16/17
Last season it was Liverpool, Barcelona, City, Juventus. You might say that there's been a drop off in the last 3 seasons, but not a big one. Mostly the difference was Liverpool and City becoming the best sides in the world.
If i had to rank them i'd say Barcelona 10/11 and 14/15 and Bayern 12/13 are the only sides who were a level above Liverpool and City