Although this is a resurrected thread, I did not comment on it the first time it was highly active. The title BEST MUSICAL has such a subjective connotation by nature, so my list is mostly a combination of personal favorites in addition to shows that I believe to be ideal to the artform.
SHOW BOAT
PORGY AND BESS
OKLAHOMA!
GUYS AND DOLLS
MY FAIR LADY
WEST SIDE STORY
FIDDLER ON THE ROOF
CHICAGO
A CHORUS LINE
SWEENEY TODD
DREAMGIRLS
SUNDAY IN THE PARK WITH GEORGE
LES MISERABLES
RAGTIME
CAROLINE, OR CHANGE
"I'm just a musical prostitute, my dear." ~ Freddie Mercury