#1 Lionel Messi

Obviously.
The best player in Barcelona's history, and possibly the best player of all time to have graced a football pitch. He might be Argentine by birth, but Messi was raised and moulded in La Masia to become the player that he is today.
Nearly everyone knows the lengths to which Barca went to secure Messi's signature - contracts signed on paper napkins and sponsorship for medical treatments for his growth hormone deficiency.
Having been given a debut by Frank Rijkaard, he quickly rose through the ranks to become the club's central figure after Ronaldinho left.
Throughout Barca's success over the past two decades, Messi has formed strike partnerships with Samuel Eto'o, David Villa, Thierry Henry and Luis Suarez, absolutely destroying virtually every defence he came up against.
At the age of 24, he became the Blaugrana's all-time record goalscorer in the year 2012. That same year, he went on to score 91 goals, breaking the record for the most goals scored in a single calendar year.
The innumerable goalscoring records that he holds would require another article entirely to list out. In spite of being primarily a midfield creator, his ball control, dribbling and finishing abilities make him one of the most potent forwards in world football.
His magic continues to weave itself at the age of 30, and he's shown absolutely no signs of slowing down. His goals-to-games ratio still remains typically inhuman. His era of personal dominance in the football world, and his personal rivalry with Cristiano Ronaldo will probably remain unmatched forever.
Messi is not just Barcelona's most important player, he is Barcelona.