It's hard to say BEST XI, since probably none of us watched Cruyff or Kubala and Maradona played here for only one season and in that season he wasn't at his best. I'll go with:
Zubizarreta
Alves-Koeman-Puyol-Abidal
Busi-Xavi-Iniesta
Messi-R9-Ronaldinho
There were many more unbelievable players like: Kubala, Suarez, Laudrup, Eto'o, Pique, Guardiola, Rivaldo, Romario, Stoickov, Valdes and of course Cruyff and Maradona. We should also mention players who were superb but didn't left such a huge mark as those mentioned above: Henry, Deco, Villa, Neymar, Marquez, Hagi, Prosinecki and that traitor Figo. When you write all those names down you come to realize how big this club actually is. Unbelievable!
EDIT: IMHO Alba doesn't belong to this TOP category. He was undoubtedly great player but many times he bottled and disappeared in big games. For me Abidal is head and shoulders above this guy. I also have to correct myself, Maradona played 2 seasons for Barca.