In my lifetime? Romario by far. Henry was also sensational in his prime...
Ever? I dunno... Pele or someone similar?
Right now? Messi I'd say. Honourable mentions: Ibra, villa, Aguero, inzhagi...
I forgot, I need to mention Norwegian legend Odd Iversen (look him up on Wikipedia!).
He scored a crazy amount in the 70s. At one point he topped the wish list for Anderlecht, cruyff was number two on the same list...