My all time favorite round is Nero's Expedition Up the Nile by Moondog. It's almost a test of a friend's musicianship- can they sing it, and will they do it willingly?
Also: I sat next to the Duchess at tea (as taught me by Kim Wallach of the Short Sisters. She also taught me Nero)
Minimalism (as taught by Nick Page)
Lachen lachen (as sung by Libana)
I've written a few as well; it's fun to try! I wrote a peace round in Arabic and another in Latin (just what the world needs: another Dona Nobis Pacem!)