I went to Milan 4 times last year. I've been there so many times in my life that I really can't tell you the exact number. 10?15?20? I really have no idea.
BUT
however
I have never seen Da Vinci's The Last Supper
At least on my 4th visit this last year I was able to go the church/monastery (Santa Maria delle Grazie) where the painting is located.
It was pretty cool.
Maybe next time I'll luck out and get tickets inside to see it.