Need your input if you've been to Barcelona

I'm travelling to Macedonia to summer vacay with the Macedonian fam. We are spending 10 days driving through Macedonia, Croatia, Albania, Montenegro, and then a couple of days at their trailer park in northern Greece.

I'm going to go to Barcelona for my last 3 days and want to know from anyone who has info, which is the best beach in Barcelona? Whatever it is, that's where I'm going to stay