I do think that Gypsy is a perfect musical - in that, as long as it is cast with people that can sing/dance it, it will always always always work, as the material is strong enough to carry itself.
I think there are only five musicals that can hold such a category, and they are:
GYPSY GUYS AND DOLLS OKLAHOMA! WEST SIDE STORY MY FAIR LADY
As John said, Gypsy is also notable for its historic importance, as are most of the musicals above. It's not one of the three most ~important~ musicals (SHOW BOAT, OKLAHOMA!, COMPANY) but it is one of the very best, if not the very best, and a perfect musical.
GYPSY is a classic to be sure. It has one of the tightest books every written for a musical, one of the most complex roles ever written for a musical and of course that overture! It is certianly in the top 5 if not the greatest. Look at the talent it has attracted to itself over the years?
-On that note I have to say that I am not a Patti Lupone fan. BUT her mama rose was just wonderful. I thought she really brough so much to the role.
There is a reason why Gypsy has been revived as often as it has. It may not be the #1 American musical, but it is close. It is extremely subjective, but it is definitely near the top if not at the top.
The truly beautiful should be lawfully restricted from wearing clothing; and the truly butt-ugly should be lawfully mandated from going naked.