My favorite has and will always be the improvised version that Jessie, James and Meowth did in "The Song of Jigglypuff" I think that was one of their firs variations of their original motto.
Some of the "Honorable Mentions" include:
Ash, Misty, Pikachu, Brock and Dulpica's rendition of the motto imitating the TRio and it was hilarious seeing them get pissed off at it
Butch and Cassidy's debut in "The Breeding Center Secret" it was the first time we learn there are other TR members who were similar to the TRio in terms of antics and having a motto. I was a bit disappointed that it turns out the TRio "stole" the motto as Meowth put it "It took weeks for us to find a decent motto we could swipe!!"
Finally in the Spiritomb episode, when Brock goes "In your ear!!" I cried laughing!!
What is your favorite version of the motto?