I agree that it is impossible to choose/agree the 'best' musical, but my favourites are:
Les Miserables
The first show I saw in the West End (and on Broadway) and the show I have seen the most number of times and still enjoy it. I do prefer, however, the version pre the cuts to reduce the total time of the show. Some of the cuts still jar and upset the scene continuity and sense.
Tanz der Vampire
I do not understand very much german but I do understand the power of a gloriously gothic, sexy, seductive musical

I love this show! (not Dance of the Vampires which I never saw live).
Wicked
Well it had to be in here didn't it?!! Had been wanting to see this show since getting the OBC and loving the music. And I loved it on stage too. I hope it has a long and successful life and also that it transfers to the West End (soon please!).
That's my twopennyworth, or should that me two cents worth?
"Well we can't all come and go by bubble"