So I've recently become a bit obsessed with the song "Franklin Shepard, Inc." even though I've never seen Merrily and know only a little bit about it. It's such a tour de force - not just because of its length (about five minutes as opposed to the usual three), but because it is a musical theatre song that is a true acting challenge with some very tough technical demands.
What other "epic" solos are similar to this in the musical theatre? You know, big acting showcases that usually involve a bit of a mental or emotional breakdown.
A few off the top of my head: "Betrayed" from The Producers, "Soliloquy" from "Carousel," "Rose's Turn" from Gypsy, "I'm Breaking Down" from Falsettos...as well as shorter ones too like "Confrontation" from Jekyll and Hyde (it is SO hard to make that song awesome instead of cheesy), "Epiphany" from Sweeney Todd...what else?
"Not Getting Married" from Company, depending on whether you consider a panic attack the equivalent of a breakdown. More recently, "Words Fail" from Dear Evan Hansen (especially if you see it on stage), which is a truly intense emotional outpouring.