""Kiss Me" from Sweeney Todd is a mouthful, mainly because both parts are intricate and then putting them together. And Johanna's is all over the place in terms of range."
I adore that song. I love how it takes place as though it were a real conversation... it's just brilliant.
Re: "Ya Got Trouble"
That is a difficult song as far as stamina goes...especially when you have to keep rehearsing it over and over again.

Another song from The Music Man that is also stamina demanding is "Wells Fargo Wagon"... though it's not that different (singing wise).
Oh...I just thought of a song... oh! "Quintet" from West Side Story. That one can get a bit tricky.
But...going back to what Rath said... it highly depends on the person. Obviously, a person with opera training will have no issues in handling a song spefically written for that genre. (well, you know what I mean.

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"I am and always will be the optimist. The hoper of far-flung hopes and dreamer of improbable dreams." - Doctor Who
"Yes, the brutalities of progress are called revolutions. When they are over, men recognize that the human race has been harshly treated but it has moved forward." - Les Miserables