The main reason I booked a tour with Viator was to visit Leonardo's "Last Supper". When I checked on the official site for the painting, there was no available time slot. One can book at the official site probably 2 months in advance. I then checked Viator, which guaranteed a visit to the masterpiece. On the scheduled day, we reached the meeting place, and our guide was waiting there for us. She was friendly and knowledgeable, and the next 3 hours she took us through the important sites of Milan, like the castle, the "Last supper", La Scala, the Emanuele Gallery and the Duomo, and explained their history. The tour includes a bus ride as well as a 1 hour walking tour. If you're visiting Milan for a day, there can't be any excuse to miss the "Last Supper", and I suggest Viator as the company has arrangements with both La Scala and the "Last supper". The tour also takes you through Milan's main sites within 3 hours, which is just enough.