When I am teaching people who like country music, I use "Cry" by Faith Hill for a good waltz. Also, though it is older, "Tennesee Waltz" still is a popular one.
But there are some nice waltz tunes hidden amongst the country music genre, which is not one of my favorites, except for when doing a waltz.
"James" by OAR, for the college set, works for a waltz as well and is a little faster tempo.
And I agree with SmoothGeezer. The slower ones are harder. "Moon River" for example, is considered one of the great waltz tunes, but it is quite slow.
If you want a good Viennese Waltz tune, try "Iris" by Goo Goo Dolls.