I'm not too familiar with some of the more obsure shows, but I think Sunday in the Park is my least favorite Sondheim. Some of the songs are absolutely beautiful (Move On, We Do Not Belong Together, Sunday, the title song) but a lot of it just sounds the same to me.
If we're talking Sondheim's best, I think Sweeney's the Father, Company's the Son, and Night Music's the Holy Ghost.
I know you.
I know you.
I know you.