I think it's pretty much impossible to really compare the bottom halves of leagues in terms of quality. For example, La Liga minnows get destroyed almost always against the big two/three, but that's also in large parts down to the often very open gameplan they employ. This might make them vulnerable against giants such as Barca or Real, but would perhaps also favor them in direct matchups with the Udineses and Stokes of this world.
That being said, if we look at the top third of the tables, where we have some kind of comparison (though also these don't mean as much as often assumed, as shown Italian, and to a lesser extent English teams not really giving a fuck about EL until this year at least), La Liga is probably the strongest, though I'd argue that the difference between all leagues are rather neglibile and often quite exaggerated, not talking about the one/two strongest teams of the leagues.