Xavi. I can't really find the right word to describe him. And if I could, there would be way too many to name. He's just awesome, he's "the General", the one that makes this team click. People talk about Messi, and rightfully so, but the attack relies heavily on Xavi's service. When him and Iniesta are clicking, I truly believe there is not a better midfield in our sport today.
I remember when Barça went through that tough run of games, where we drew at Betis and at Lyon, lost to Espanyol (unfairly), and then to Atlético. It coincided with Xavi getting fatigued, you could easily tell he wasn't as fast or as sharp as he usually is.
When he's on, this team is, well, to put it simple: deadly.