If the 20 Prem teams played the 20 La Liga teams it would be very interesting, and I think it would be about even (of course whoever was officiating would make a big difference, the stricter La Liga refs would give the La Liga teams an advantage, and the English refs more accustomed to a physical game would give the English teams an advantage). The La Liga teams would struggle with the pace, physicality, and directness of the English teams. The English teams would struggle with the skill and passing of the Spanish teams. It would be very even. As it would be if all of the teams from either of these leagues played all of the teams from Serie A or the Bundesliga.
---------- Post added at 07:03 AM ---------- Previous post was at 07:03 AM ----------
Ka5a8ian brought up a very good point I was going to mention: since its inception, the Prem has had fewer winners than La Liga, and since the mid to late 90's it's always a one or two horse race. It used to be a one-horse race with ManU always winning, then it became a two-horse race with ManU and Arsenal, then Arsenal started its selling policy and Chelsea came into the fold. Since then it's been a two-horse race between ManU and Chelsea, and this season after Chelsea's surprise start (I along with many others thought Chelsea was too old to compete for a title this year) it became a one-horse race as ManU was the only team with a real chance to win it. Traditionally over the past decade, the Prem has 3-4 very good teams while the rest suck (this was the case in the mid 00's until last season, when the same Big 4 would finish in the top 4 every year, they were good teams that would all do well in the Champions League, which people would cite as proof of the Prem's superiority, while the rest of the league was terrible). La Liga usually had two very good teams and 3-5 very solid teams, none of whom were quite as good as Arsenal or Liverpool but the rest of the league was stronger from top to bottom. But people would say "well, the Prem has the best teams so it's the best league."
Now the tables have turned and it's the other way around, with La Liga having the best teams but the Prem having more depth, but people say the lower-tier Spanish teams are bad (which is untrue). The biggest difference between La Liga and the Prem is that La Liga's top two are much better than the Prem's top two (although if Barca makes it to the final, ManU should beat them since their style of play is the perfect antidote to Barca's) and Barca/R. Madrid consistently beat up on weaker teams, while ManU has been very inconsistent this season (as Chelsea and Arsenal have been, which is why they couldn't compete for the title). I'm tired of hearing people make the SPL comparison because it's so untrue (especially considering that one could make the same comparison for the Prem, since usually ManU and possibly Chelsea are the only teams with a shot in **** at winning the title). The EPL fanboys conveniently forget that in 08-09, La Liga had SIX Champions League caliber teams, and the worst of the 6 (finished in 6th place in La Liga that season), Villarreal, played ManU twice and drew twice as the sides were completely equal and cancelled each other out.
The past two seasons, both R. Madrid and Barca have been incredible and can't be caught by the other good Spanish teams, who have struggled recently, especially due to financial problems for the reasons mentioned above (TV deals). But the struggles of Atletico Madrid, Sevilla, and Villarreal the past two seasons (although Villarreal has been very good this season) should only testify to the depth of the Spanish league. Look at Atletico Madrid: they are absolutely stacked with talent. They have two of the best top 10 strikers in the world, two very solid attacking midfielders (Reyes and Simao, who left but has been replaced by Juanfran), and two very solid center-mids (Paulo Asuncao and Tiago, both of whom I think are very good). They have a great young keeper and a very solid defense with Godin, Ujfalusi, and Dominguez in the middle and Filipe on the right. They are more talented than Tottenham IMO, and in 08-09 were a great team, better than this year's ManCity. Yet they are doing very poorly in La Liga. Why? Because La Liga is very deep, much moreso than the Prem fanboys will admit, and Atletico Madrid constantly loses to these strong, lower half teams.
IMO there is very little difference in the overall quality (as in the quality of every team in the league) between Serie A, La Liga, the Prem, and the Bundesliga. La Liga's top two teams are the best in the world, not much in terms of the 3-5th best teams, and a very strong league other than that. The Prem has 5 very solid teams. Serie A and the Bundesliga don't have any very good teams this year (Inter has tons of talent but has had lots of injuries and very poor coaching), but the overall quality of the league is very strong. In Serie A you have outsiders like Lazio and Napoli who occasionally have great seasons other than the big 4, who have struggled recently (due in no small part to the depth of Serie A). The Bundesliga doesn't have one world class team other than maybe Bayern Munich (who has struggled this season in the league due to the fact that unlike the other three leagues, there are no easy matches) but has the most parity. That's why every year there is a smaller team that challenges for the title: Hoffenheim, Wolfsburg, and this year, Dortmund. I'd say that the bottom half of the Bundesliga right now (which includes Schalke) is better than the bottom half of any league in the world.
And of course Ligue 1 has improved in recent years too...look at how talented Lyon has been the past couple of seasons; they can't win a title there. This year's title race there is completely wide-open between about 5 teams, and it's the only league out of the 'Big 5' where we don't know the winner already (I think we can safely bet AC Milan will take Serie A).
So yeah, I don't think there's anything separating the Big 4, and Ligue 1 isn't that far behind. The Argentine and Brazilian leagues, although they don't have any particularly good teams (they can't hold onto any of their players with no Champions League and extremely low wages, particularly in Argentina) the leagues are very strong from top to bottom. I'd say that the bottom half of those two leagues are just about as strong as the bottom half of any of the Big 5.