Team for team, it's the biggest league. It's just that Real Madrid and Barcelona happen to be much bigger than any other clubs on the planet. They come in for your player, and he's probably going to leave.
That's the case whether you're Liverpool, Manchester United, Bayern Munich of AC Milan.
Perhaps the only viable long term option is to avoid possessing a single truly great individual and build a monolithic squad like Chelsea or Man City - neither contains a player the aforementioned Spanish giants would jump through hoops for, but they have the depth to ensure that they're never going to be left behind regardless of who departs.