Couples with chemistry fire up the screen with lust and intensity and make our hearts ache in compassion and yearn in recognition, and I think onscreen chemistry comes from a combination of things: the actor's must have been in love and experienced a broken heart, or have the ability to act like they have, and there should be some mutual attraction, but it usually can't be acted upon offscreen without destroying the spark onscreen. Onscreen chemistry between actors is a defining factor in whether a film is succesful or not, and there have been many a terrible pairing in film: Jennifer Lopez and Joseph Fiennes in Maid in Manhattan, or Matthew McConaughey and Sarah Jessica Parker in Failure to Launch (which was redeemed by the wonderfully kooky Zooey Deschanel). Here, I've compiled a list of the onscreen couples with the most chemistry, in my humble opinion:
5. Uma Thurman & Bryan Greenberg in Prime

4. Claire Danes & Leonardo Dicaprio in Romeo & Juliet

3. Nicole Kidman & Jude Law in Cold Mountain

2. Angelina Jolie & Brad Pitt in Mr & Mrs Smith

1. Diane Lane & Olivier Martinez in Unfaithful

Also notable: Nicole Kidman & Ewan McGregor in Moulin Rouge, Diane Lane & Viggo Mortensen in A Walk on the Moon, Leonardo Dicaprio & Kate Winslet in Titanic, and Heath Ledger & Jake Gyllenhal in Brokeback Mountain.
Any hot onscreen couples I missed? Or terrible ones?