Well I've been many times to Camp Nou and it's amazing, especially during Champions League nights. However, I must mention San Mamés, the stadium of Athletic Club de Bilbao. The stadium itself is nothing special, but it's a legend, and this club has one of the best supporters in the world. The crowd is always supporting the team even when they lose. They are one of the only clubs that has never been to 2nd division in Spain. Their stadium is one of the oldest in Europe, it opened soon after Old Trafford. Very special place... but sadly, they are demolishing it to build a new one, maintaining some elements.
I'm a Barça fan and have been to the Camp Nou many times but the Bernabeu is intense. Stadium aside if you want the best crowd go to an Atletico Madrid game.
Hello to everyone. The best stadium is New San Mamés, the stadium of Athletic Bilbao. I also would mention Cornellá, from Espanyol. The best supporters and atmosphere is in Vicente Calderon, the stadium of Atletico de Madrid. Other remarkable supporters are Athletic Bilbao fans and Cadiz fans. Barcelona and real Madrid stadiums (Nou Camp and Santiago Bernabeu) are not so remarkable, unless you just appreciate high capacities. Those stadiums are the largest in Spain and can be impressive somehow. However, supporters are quite desperate and mean when their team looses.