No, I dont think there is any data that succinctly sums up 'quality of football' in one metric. The only one I'd jump to is the number of different clubs that have made the CL from a country in the last decade - where England leads with 4 different clubs, behind Spain (3), Germany (2), Italy (1), France (1).
I'd bet the stat would be true if you look at the semis as well, but I cant be fucked going back and doing that by hand.
I'd wager the stat holds true if you go back 20 years even. Arsenal and United have all qualified in that time - from Italy its been the same 3 clubs since Sampdoria in 1991? and Spain since Valencia 01?
Put another way - if you dont think the Premiership is the best league right now, what do you think is?
The data would reflect the distance covered, making it 'harder' to play there because it's "faster" or whatever.
There's no such thing as "best league" because leagues aren't entities, they carry separate entities. The only time, in like the history of the sport, we have had a clear answer there was in the late 80s to early 00s with Serie A. The PL being born in that environment gave them the trigger that they have to become "the best", but arguably they were behind other top leagues as a collective in various parameters until this day.
What exactly is the argument that the EPL is the best league at the moment? The CL "dominance" because of two all-English finals? Two of those were flukes, I don't see people saying La Liga was the best league in the early 2000s because they had Real Madrid and Valencia in the CL final so often, and it was much tougher to get there at the time.
The only thing that typically gives this statement is having the best players and teams. Their PR machine is just too strong, but even the strongest marketing campaign cannot make what we saw as the "Ballon d'Or shortlist" the most laughable thing in world football yet!
The very best players are still in other leagues. PSG attracts the best talent, Bayern and Real Madrid have the best players and Barcelona despite being useless for years still attracts better players. Heck, even Juventus and I'd argue even Inter attract top names once again. Sure, the PL guys can flex their wallets, but their shopping cart isn't taking them to Bond Street level football players.
Being a top heavy league apparently was bad, but now it's good? This is why you can never win with PL marketing. Scoring lots of goals = good. Then Serie A has more goals per game as opposed to EPL for like 50% of the time since its inception but somehow that's bad now and a counter-argument. There's no winning with PL fanboys and being surrounded by this infrastructure brainwashes people, even if they don't suspect it, even if they don't care much about the league.