I don't think UCL winner by nationality is a place to tell whether the league is best but more team from domestically and consistency is important to analyze which league is best. I feel like EPL has a lot of better team, spreading evenly quality over the league. There's the reason why EPL is the only league that got 4 teams in quarter-final three times, not even La Liga, Serie A or Bundesliga has managed that and were first to have 5 teams involved in the round of 16 last season and could have another 5 again in round of 16 next season if Chelsea qualify for UCL via Europa league. EPL had 3 teams back to back in the semi-final so 9 semi-finalist come from PL in just 3 years, they are looking likely to get 3 teams involved in the semi-final again, that's more record than any other top 5 league combined who had 3 teams in the semi-final. EPL had a more different winner than anyone and had 7 different teams involved in a round of 16 over the last 2 seasons since 2016.
So, EPL hasn't had the best team that can challenge Barca/Madrid especially having Ronaldo and Messi, they have advantages over any Serie A, EPL, Bundesliga. Messi and Ronaldo are coming to the end of the era in the 2020s, dominance is up for grab, PL is more likely to take it back, Serie A looks strong too but needs consistency.
La Liga was best for 5 years after Bayern's UCL winner because of the many teams La Liga represented went on to win European trophies and dominate Europe consistently.
So if Bayern dominates UCL for 5 years and winning 5 UCL titles in a row, would any of you consider Bundesliga to be the best league because of Bayern? I'd consider them to be the best league if they had Dortmund, Schalke, Bayer, etc to win European trophies and reach semi-final and finalist every season for 5 years or even 3 years. One team dominating Europeans trophies doesn't mean their league is best because they don't have other teams can dominate Europe. Porto can dominate the Europeans for years but still won't consider their league to be best, they need Benfica, Sporting, etc to win trophies, reach semi-final/finalist consistently to back up a statement regarding of whether their league is best in Europe or not.
So, if EPL wins UCL this year, I still wouldn't consider EPL to be the best league because of one team or just one season but EPL needs to have more teams of dominating Europe consistently to be considered as the best league in Europe.