There are so many. Let's go with "Who is your favorite... and/or who are
some of your favorite couples?"
My favorite film couple is Daniel and Lauren in "A Little Romance," a somewhat little-known gem.

1979, with Diane Lane as Lauren. She was 14 at the time and was incredible. The kid who played Daniel was Thelonius Bernard and was equally amazing. You get Sir Laurence Olivier, Sally Kellerman, and Broderick Crawford, too. A must for true movie buffs
and it's a really great story.
A Little Romance (1979) - IMDb
Bogey and Bergman in "Casablanca"
Matt Damon and Minnie Driver in "Good Will Hunting"
John Cusack and Ione Skye in "Say Anything"
Leonardo DiCaprio and Kate Winslet in "Titanic"
Ethan Hawke and Julie Delpy in "Before Sunrise" and "Before Sunset"
and Kermit and Miss Piggy in all their films