Using the tern 'best' does depend on personal taste, so one person's best musical, may not be another's. Maybe the best way to judge is to look at commercial success, and using that criteria, the most successful in terms of longevity, and therefore 'best' has to be "Cats" which has run in theatres around the world for nearly twenty years.
Personally I think his music is unbearable, especially the newer stuff. It shouldn't even be played in supermarkets!
Still, there is one of his musicals I listen to sometimes: it is "Jesus Christ Superstar". So if it HAS to be Webber, try that one.