This is a basic answer on my part but John Krasinski and Jenna Fischer were were unbelievably natural, their attraction felt so real it's hard to believe they were nothing more than friends in real life.
Julie Andrews and Christopher Plummer in The Sound of Music also had such a believable tension.
David Schwimmer and Jennifer Aniston come to mind as well. What other examples can you think of?
Edit: I forgot to add Ethan Hawke and Julie Delpy because jesus christ i can't fathom them not being in love