The best chemistry between two actors

Okay, I'm going to go with Kajol and Shahrukh Khan in Kuch Kuch Hota Hai (and all their other movies as well). I'm usually not a fan of romcoms, but their chemistry really made the movie work for me.

Cage and Travolta in face off.
Jeff Daniels and Jim Carrey in Dumb and Dumber

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

Robert Redford and Paul Newman in Butch Cassidy & The Sundance Kid (1969)
Jodie Foster and Anthony Hopkins in Silence of The Lambs (1991)
Robert De Niro and Charles Grodin in Midnight Run (1988)
Steve Martin and John Candy in Planes, Trains & Automobiles (1987)
John Turturro and John Goodman in Barton Fink (1991)
John Travolta and Olivia Newton-John in Grease (1978)
Woody Allen and Diane Keaton in all their films together

Raul Julia and Anjelica Huston - The Addams Family.
Also Tom Welling and Michael Rosenbaum - Smallville.

Any film where the two became a couple.

Jack Lemmon and Tony Curtis in Some Like It Hot, one of my favorite films of all time.

Crowe and Bettany in Master and Commander...they couldn't stand each other half the time but each respected the other