Not exactly full songs, but the two most difficult pieces of music in SWEENEY TODD are the search for Tobias ("Toby, where are you, luv?") and the confrontation before the oven waltz at the end ("No, not lied at all/No I never lied"). Mrs. Lovett and Sweeney are both singing in different time signatures and Mrs. Lovett has a breathless rush of tricky rhythms to spit out, while Sweeney has to count awkward rests and emote at the same time. I've done the show twice, and a disproportionately large chunk of music rehearsal time had to go to those two twelve-measure bits, because it's fiendishly difficult for the two singers to end at the same time.
I'm going to offer a couple of unexpected choices--Sister Amnesia's puppet number, "So You Want to be a Nun" in NUNSENSE--she has to switch from a low, hoarse character-belt voice for the puppet to a crystalline operatic tone for herself in a fraction of a second.
Also, the entire score of FOREVER PLAID. Twewnty-seven songs all done in four-part harmony. It's all completely exposed; no one can go off-pitch or stick out of the blend once without being noticed, and everyone has at least a few harmonies that are absolute bitches--no one is always on melody, which is all some tenors (and sopranos) know how to sing.
I ask in all honesty/What would life be?/Without a song and a dance, what are we?/So I say "Thank you for the music/For giving it to me."