There were six duets* featuring Rob and Laura during the run of the series (I wish there had been more). Here’s my personal ranking (ordering them is not easy, as they’re all great):
6. Wearing Santa Claus outfits, during “The Alan Brady Show Presents.”
5. Performing “Harmony” at Herbie’s Hiawatha Lodge in “The Secret Life of Buddy and Sally”.
4. “I’ve Got Your Number”, in “The Alan Brady Show Goes to Jail”. The duo is dressed in prison outfits, and does a tap dance with their ankles joined by a chain.
3. “Carolina in the Morning” – “The Sam Pomerantz Scandals”.
2. “Mountain Greenery” (this is the only one that is singing only, no dancing; but I rate it highly because of the up-tempo jazz arrangement and nice harmony. Performed in the episode “The Sleeping Brother”.
1. “You Wonderful You”, in the early episode “Oh How We Met on the Night That We Danced”.
* Rob and Laura did perform one other song and dance together, in the episode “Too Many Stars”; but I have excluded it from the list because it was a trio, with a non-recurring character.