Best CL side ever. They haven't done enough domestically.
Most succesfull CL side ever is the right way to put it. Not best, because the best team might not win the CL because of numerous uncontrollable factors. Bayern for example was better in CL than Madrid this season.
Best is always subjective.
Pep's Barca has the most arguments:
- best individual season in 2009 winning 100% of the titles they played
- domination in La Liga winning 3/3 titles including 2 La Liga - Copa doubles
- constant humiliations of their arch rival, including historic beatings like the 6-2 and 5-0 both home and away
- great domination in CL, winning 2/3 titles in that time frame
- unique style of play that influenced future generations of players and managers
- some of the greatest players in their positions to ever play the game at the same time on the pitch
- 3/3 ballon d'Or finalists when at their peak
- only team to basically dominate almost every game they played, to the point that other great european sides look pub teams.
No team has that many claims than that Barcelona.
Madrid already lost in many of these areas. They didn't dominate La Liga, they were mostly 2nd best to Barcelona in the last 3 years, losing 5 out of 6 domestic titles. They are a great CL specialist team.
Madrid could get better results in CL than prime Pep Barcelona (if they win tonight) because they were luckier (2016 CL for Madrid is an incredible series of lucky events going in their favour) and because they basically don't fight on all fronts with the same intensity. Also because they are a great team as well.
This Madrid isn't even that much better than this Barcelona, only marginally. 2011 Barcelona would have beat them like they beat Mourinho's side.