We have released a list of the top 1000 works in WorldCat. This list is unlike some other lists in that we have consolidated the various manifestations of a work as best we can and factored that into the ranking. To do this we used our FRBR work-set algorithm with some subsequent manual editing. We have also provided links into Open WorldCat.
As you would expect, the list is heavily loaded with classics. These will tend to have both high holdings (they are in many libraries), and many manifestations (they have a rich and varied history of publication and augmentation).
Garfield makes a showing at number 18. I was surprised that Beowulf figured so highly. There are also lists for particular work types. Huck Finn tops the fiction list. The Messiah is the highest musical work.
It can be addictive!