We don’t know which the best team in Europe for 2017-18 is yet but we do know where the best league is: England. There is a weighting issue but we think the decision can be made now.
Sidebar: The weighting issue is how you weight the group stage versus the knockout stage of the Champion’s League (CL). An imprecise analogy is the NCAA basketball playoffs. If your bracket has every game but the last one right you will probably lose because most of the games weight the final game heavily (see here). The CL competition is different from the NCAA because the group stage involves four teams playing six games home and home versus common competition. Still if you weight the championship highly enough then the story isn’t over.
We think the story is over because of all the data. Seven countries qualified multiple teams with England (and Wales if you want to be precise) leading with five and Spain second with four. There are eight groups of four with the top two in each group qualifying for the next stage. Since none of the countries with two or three qualifiers sent all of them to the next stage we can limit our discussion to England and Spain. England had four group winners and one second. Spain had one group winner, two seconds, and one third (not qualifying for the next round). Advantage England. What happened in groups with English and Spanish teams? Three groups had Spanish and English teams. It all three cases the English team finished ahead of the Spanish team including the Spanish elimination.
Was the English group domination happened because the strongest English teams met the weakest Spanish teams? Liverpool (currently fourth in England) beat Sevilla currently fifth in Spain). Chelsea (third in England) beat Athletico Madrid (third in Spain). Tottenham (sixth in England) beat Real Madrid (fourth in Spain but the defending CL champion). In short, the England-Spain competitions seemed reasonably fair if not favoring Spain.
We think it is more likely that a Spanish team will lift the CL trophy but the best league should be heavily weighted on the large number of observations in the group stage. Spanish teams would have to dominate the knockout stages for us to reconsider the 2017-18 league ranking.