Wolfgang is one of the greatest mathematicians of the 20th century, and has made profound contributions to number theory, particularly on diophantine approximation. His series of amazing papers on irregularities of point distribution in the 1960s and 1970s, together with his lecture notes on the subject, has expanded the boundaries of the subject from the earlier pioneering work of Roth.
Most important of all, Wolfgang is a very modest gentleman, and is extremely generous and helpful to his colleagues. I am extremely privileged to have met him while I was still a graduate student, and he has given help and encouragement all these years.
I recall visiting the University of Colorado in January 1983 at his invitation. There were three other visitors. After four days of seminars and discussions, someone suggested visiting the ski slopes on Friday, and Wolfgang greeted this with great enthusiasm. His effort at teaching me to ski was not so successful, although I did not come to any serious harm.
Wolfgang's love of chocolates is legendary. For the 1985 analytic number theory meeting in London which I co-organized, I indicated on the invitation letter to him that chocolates would be provided. Wolfgang's letter of acceptance arrived very quickly.
One of Wolfgang's amazing results in irregularities of point distribution concerns the large discrepancy of convex sets. The 2-dimensional case in particular is very enlightening, particularly so if studied from the viewpoint of chocolate cakes, as one will no doubt see if one clicks here.