Pange Lingua for me.
For my children The King of Love My Shepherd Is. They all had to sing this a cappella. Once, they sang it for a Requiem Mass and they've not forgotten how moving that was.
This thread wouldn't be complete without a link to http://www.mgilleland.com/music/moratorium.htm
Someone recently pointed out to me the musical similarity between "Here I Am Lord" and the theme song of "The Brady Bunch" --- I haven't been able to get that out of my head since!