John Travolta and Olivia Newton-John guest on "Ellen": All-time best on-screen couple?

Yesterday one of the most iconic on-screen couples, John Travolta and Olivia Newton-John, dropped in on "Ellen" to talk about their new Christmas album. "This Christmas" will feature stellar guest appearances by Barbra Streisand, James Taylor, Tony Bennett and Kenny G.

Travolta and Newton-John starred together as iconic screen couple Danny Zuko and Sandy Olsen in "Grease" — but who's your favorite on-screen couple? Is yours from one of the classics, like Humphrey Bogart and Ingrid Bergman as the doomed Rick and Ilsa in "Casablanca"? Or real-life couple — and the subjects of the Lindsay Lohan's movie "Liz & Dick" — Richard Burton and Elizabeth Taylor in “Cleopatra"?

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And let's not forget the smoldering tension between Clark Gable's Rhett Butler and the unforgettable Vivien Leigh as Scarlett O'Hara in "Gone With the Wind."

Then, from the romantic-comedy era of the '80s, how about Harry and Sally in...well, "When Harry Met Sally..."? Or the steamy romance between Baby and Johnny in "Dirty Dancing"? And in the '90s, there was the tragic Jack and Rose in "Titanic," and "Pretty Woman's" odd couple — millionaire Richard Gere and Julia Roberts' call girl.

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Who's your all-time favorite on-screen couple?