BenElliott said: "...Passion is another. I absolutely love this show. I'm probably the only person in the world that does, but I think it's gorgeous. It's weird, disturbing, gross, confusing, and yet it all comes together in a magical Sondheim soup with a dash of Donna Murphy. I would actually argue that this is Sondheim's most beautiful score (not necessarily his best lyrics, but certainly his best score). Meanwhile, I think nearly everyone in the world not only hates this musical, but loathes it...."
You are NOT the only one. I didn't mention it because it played on Broadway 25 years ago, outside the delimiters of the OP's question.
But I was astonished and moved by its absolute refusal to cater to traditional musical play tropes such as "love conquers all", "love makes the lovers beautiful", "it is always better to be loved than not". Hell, even the set was (intentionally) ugly!
(You question whether PASSION has Sondheim's best lyrics, but I would counter that it has precisely the lyrics Sondheim thought it needed. Except for "I'll Say" and its reprises, Sondheim eschews the cleverness that made him famous because he believes wit makes characters sound intellectual. PASSION is about people who follow their hearts, not their reason.)
PASSION competes with MERRILY for a place in my five, favorite Sondheim scores. (Yeah, I know they are entirely different, but I'm assuming I can only take five recordings to a desert isle. After FOLLIES, COMPANY, NIGHT MUSIC and SWEENEY, that fifth slot is a tough one.)