The poll options are all over the place. But I guess if you have to make a top 5 you have to consider several things like legacy, loyalty, trophies won etc. My personal top 5 would be:
1.Messi
2.Xavi
3.Iniesta
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4.Cruyff
Barcelona was in the mud before Cruyff's arival in the mid 70s. They havent won the league since 1960, won a copa here and there, Sevilla kinda levels. He also won 2 Ballon D'ors while here, spent most of his prime here.
5.Ronaldinho
One of the main reasons for Barca's revival after the late 90s-early 00s disastruous running of the club. Gave a huge boost to the club both sportingwise and financial. The team gained a lot of fans during that period because of Dinho, and he ultimately started Messi's career in a symbolic way.
Other players who deserve mentioning: Puyol, Koeman, Busquets, Eto'o, Stoichkov, Laudrup. Could also mention older players I guess like Kubala, Luis Suarez(the spaniard who won the Ballon D'or).
Ronaldo Nazario reached arguably his peak here, but his Barca career is too short to be mentioned in a top 10. Likes of Romario and Rivaldo also reached peak of their career here.