Robben played for the Manchester City manager at Bayern.
He told BILD: "It is a difficult decision, I have lived many beautiful things with many coaches. But if you ask me who is the best, I say Pep Guardiola. As for football, he is brilliant.
"When Pep arrived I was already 30 years old, at that age you just try to be consistent and not get worse. But at his command I was even better. Suddenly I played in other positions where I never thought I would play. He was the best."