After listening to a small selection of the "best" birdsongs, most agree the Common Koel is the top choice in this field. Something I wondered after looking at the graphs through the mediaplayer software is if a lie detector would pick up traces of deception among the birds as it would on normal human voices... It would be very interesting to see in practice.

The image above is credit to graemechapman.com.au who seems to know a lot about the birds on the website mentioned.

A sample of the bird song of the Koel is available at:- http://birdsinbackyards.net/images/audio/eudynamys-scolopacea.mp3