The Polish star has started his Barca career brilliantly, scoring eleven goals and providing two assists in eight competitive matches.
Former Barca coach Van Gaal said, "I think Robert Lewandowski is the best striker in the world."
The Dutchman also commented on Frenkie de Jong's form at Barca.
He added: "Frenkie has done very well, hasn't he? I think he seems very relaxed, with a lovely smile. Fantastic. I admire him."