Sweeney Todd's "Epiphany" was the first thing that came to mind, for me... for showing a so clearly-deranged character's mental state so profoundly, energetically, and disturbingly.
I'll also second "Is Anybody There?" as a tremendous work-out for an actor, certainly less mental-instability in that one but an emotional freight train of determination, pride, and commitment(!)
And yes, "Soliloquy" from Carousel is a pretty unmatched epic, though it's mood swings and emotional tones are more drawn out and paced, it is unquestionably an epic journey.
Sadly, "The Ballad of Booth" from Assassins isn't a solo, because it's content is right up there with "Rose's Turn," but it's form makes it an imperfect equal.
And, if I may be unconventional, "Work the Wound" from Passing Strange, at least how Stew did it, was a heck of an emotional work out.
Words don't deserve that kind of malarkey. They're innocent, neutral, precise, standing for this, describing that, meaning the other, so if you look after them you can build bridges across incomprehension and chaos. But when they get their corners knocked off, they're no good anymore…I don't think writers are sacred, but words are. They deserve respect. If you get the right ones in the right order, you can nudge the world a little.
