First and foremost, I really don't think the English premier league is the most competitive in the world because if it was that competitive, Manchester city wouldn't be dominating the English premier league the way they have been dominating the league for year now.
What I see that is actually going on in the English premier league is that apart from Manchester city that is a level above the rest, the other top teams are just competing with each other, and to be honest here, this current league standings can't be maintained till early January probably, by January latest, we are definitely going to see the real challenger of Manchester city.
Then talking about the English premier league as the strongest as you claimed, I totally disagree to that, yea they have Manchester city right now, the most competitive team in the uefa champions league in recent times, but that doesn't make them the strongest because the likes of real madrid, Bayern Munich and even inter Milan are still in the picture presently, so in my own opinion they are just one of the best, not the best among them all.
In competitiveness, I think Premier League is the most competitive because Manchester City didn't win easy, even though they've become a champion four years streak. Liverpool and Arsenal can give a fight, sometime Chelsea and Newcastle also joined the race.
In Serie A, it's hard to predict the champion, but it doesn't mean it's competitive because their quality are keep dropping.
Yeah of course La Liga is the strongest league, but it's not competitive since either Real Madrid or Barcelona who become the champion.