These discussions are always tough because in most cases the leagues are just different from eachother, not better or worse.
That said, I really don't think it's a better league than the Premier League. Maybe four or five years ago when the likes of Juventus, Milan and Inter were dominating Europe, but these days there's only really Inter who I'd identify as being in the top bracket of clubs in Europe.
I was really pleased with Milan's performance against United on Tuesday, because it dispelled the myth that the Serie A is slow etc, but the reality is that Milan, the second-best team in Italy, succumbed to a pretty suspect United side who had an arguably poor game. And Inter, for all their domestic dominance, were completely outclassed by Liverpool and United in the past two Champions League campaigns.
I think previous comments about the overall quality in Italy being better than in England are accurate, and you can certainly say the same for La Liga as well. I think if you put Wolves against Udinese or Racing Santander, they'd get absolutely embarassed and completely outclassed. But ultimately I don't think you can say the Serie A is a better league when the top quality players and teams simply aren't there anymore.