Agree it will be great to see him in these tests, wiley and technical group of midfielders Madrid has. Looking forward to his reaction to Atleti too, a different set and style of outstanding midfielders--though I know much later. Hard for me to fathum that large of test via Lyon. One or two of City & Liverpool & Bayern will be his ultimate tests this year I believe, Madrid is better in midfield but those teams better collectively squeeze you and come at you quickly from different directions.
You're right those teams are better collectively, and City's midfield might be as good as RM's talent wise.
Liverpool and Atletico would be good tests for Arthur because he'd have to face the most energetic midfielders. Liverpool's midfield is the best at pressing high and playing quickly up front. But we already know that Arthur does brilliant when he's pressed. Still those 2 opponents would be a good test to see how good his gas tank is and whether he can perform well for 90 mins against those workhorses.
I believe RM and City are the 2 ultimate tests for Arhur because he'll face midfielders equally as good technically and talent wise as he is. Plus we know for a fact that in a Clasico and in a potential match/tie vs City, these 2 teams will put on their best effort in terms of work rate even if that means they'll have to overperform for 2 games.
Regarding Bayern, their midfield and their team more generally hasn't been that impressive this season. We'll see how they cope against Liverpool in the round of 16, they'll get a huge boost if they go through but Liverpool are the slight favorites imo.