I find the most taxing songs to be ones that stay in the upper range for pretty well the whole song, like Hotel California. HATE doing that song. :-)

Range wise I would have to say that songs by Mariah can be challenging as she loves to sing through her whole range in every song.

Celine Dion is probably the most difficult to sing well as she is so sickeningly perfect in her trills and dynamics that if you don't match her, you will automatically suffer in comparison.

But all in all the most difficult songs for me are ones that use any type of low falsetto. Like "How Deep is your love", "Tiny Dancer", or the last note of Meatloaf's "Heavan can wait". If my voice isn't in tip top shape than the first thing to go is my falsetto, and the low range is gonzo for sure.