I find different songs present different kinds of vocal challenges:

1) Songs with a wide vocal range
* Star Spangled Banner

2) Songs which contain unusual musical intervals:
* I Love You, Cole Porter ( descending Maj 7th interval)
* Lush Life, Billy Strayhorn (unusual intervals)

3) Songs with off-beat or sycopated rhythms or unusual meter changes
* 'Anyone Who Had a Heart', Burt Bacharach: 4/4 & individual measures of 5/4
* 'Top Hat, White Tie, and Tails', Irving Berlin: tricky rhythm & sycopated phrasing

4) Fast tempo songs with tricky phrasing & lyrics (tongue twisters)
* 'One Note Samba', Antonio Jobim
* 'Joy Spring', Clifford Brown
* 'I've Been Everywhere', Hank Snow