I'd like to add a vote for Sweeney, and add a sympathy plea for Parade... which reminded me of Sweeney quite a bit in the way the score reflected the tone of the story.
It seems that there are two definitions of "seamless" in this context: (1) an effortless-looking integration of book and score, so that text and music flow between one another, and (2) a book and score that do not "morph" into one another, but that reflect each other's tone so magnificently so as to appear to have been written by one person [which, by definition, should discount shows written alone]
I would cite Music Man as a fine example of (1) and Gypsy as a "template" example of (2)
...or maybe those really aren't two terribly different things, after all. Sitting on the fence feels so good.
"There are only two tragedies in life: one is not getting what one wants, and the other is getting it." -Oscar Wilde