The Best/Worst performances by an actor/actress

[msmpls]

Will Farrell:
Best - Old School
Worst - Taladega Nights

Johnny Depp:
Best - Edward Scissorhands
Worst - The Astronaut's Wife

Kirsten Dunst:
Best - The Virgin Suicides
Worst - Spiderman

[mrbasehart]

Marlon Brando.
Best = A Streetcar Named Desire
Worst = Island of Dr Moreau

Eddie Murphy
Best = Dreamgirls
Worst = Norbit

Naomi Watts
Best = Mulholland Drive
Worst = Tank Girl

[Minnesota]

Paul Walker
Best - Running Scared
Worst -  Timeline

Vin Diesel
Best - Pitch Black
Worst - The Chronicles of Riddick

Joaquin Phoenix
Best - Walk the Line
Worst - Gladiator

Val Kilmer
Best - TombStone
Worst - The Island of Dr. Moreau

Jennifer Connelly
Best - Dark City
Worst - Blood Diamond

John Travolta
Best - Pulp Fiction
Worst - Battlefield Earth

... I better stop here ... fun game, you can literally to this to EVERY first actor / actress that pops in your head

[msmpls]

Matt Damon:
Best - The Talented Mr. Ripley/Dogma
Worst - Stuck on You

Ben Affleck:
Best - Dogma
Worst - Every other movie he's done (save for maybe Good Will Hunting)

[mrbakasan]

Michael Madsen:
Best - Reservoir Dogs
Worst - Bloodrayne/Sin City (loved Sin City, but he was awful)

Tom Sizemore:
Best - Heat
Worst - Ring Around the Rosie (I'm pretty sure he is drunk through the entire movie)

[Grillslinger]

Uma Thurman
Best: Kill Bill
Worst: Batman and Robin