Don't know about the best because I have only one:
WONDERFUL WORLD
2003
Multi-Grammy winners Tony Bennett and k.d. lang duet on a selection of songs inspired by the legendary Louis Armstrong, including "What A Wonderful World", "La Vie En Rose", "Exactly Like You", "You Can Depend On Me."
Lee Musiker- Piano
Paul Langosch- Bass
Clayton Camern - Drums
Gray Sargent - Guitar
Special appearance by Scott Hamilton - tenor sax
http://www.wbr.com/kdlang/wonderful.html
it is with KD Lang, released 2003. I find it very enjoyable but don't know how it compares to the rest of Bennett's huge discography.
Oh, if you want to side track a little, have you listened to Paul Kuhn?
http://www.inandout-records.com/records_detail.php?artist=00000001&record=00000001&mode=
That one is fun too :-)