I couldn't begin to choose just one, but certainly one of the most beautiful songs is Eva Cassidy's version of "Somewhere Over the Rainbow." Yeah, yeah, I know...that song is just too trite & cheesy to take seriously, right? Well I dare you to listen to her sing it and then say that. No matter how often I hear it it still gives me goosebumps!
Btw, "Wade in the Water" & her cover of Sting's "Fields of Gold" from the same album are also amazing. Truly an incredible voice, taken from us far too soon.