Currently spearheaded by the front three of Messi, Suarez and Neymar (perhaps the most fearsome strike force ever assembled), Barcelona have returned to the top of the footballing world after a trophiless 2013/14 campaign. Winning a league, cup, and Champions League treble has once again cemented Barcelona as the best team in the world, capable of attracting the biggest names in football to play at the Nou Camp. But, throughout its illustrious history, the Catalan side has also let some of the globe's most renowned players slip through their fingers - often leading directly to a multi-season slump. Some were tempted away by lucrative wages, while a surprising number left after a fallout with the club's manager - or even ownership. Many were unable to fully impress in such a star-studded environment, yet cemented themselves as heroes elsewhere. Horrifyingly for Barcelona fans, a number of Real Madrid's finest players have ended up in the nation's capital by way of the Nou Camp, proving that even the best team in the world can lose a true gem to their greatest rivals. Here are ten players Barcelona should never have let leave. Had these names been kept around, the duopoly enveloping La Liga (and arguably European football as a whole) could well have become a one-horse race.