I'd probably say her best performance was in Pure - definitely showed off her acting chops. But since it was a minor role, her character Lara in Dr. Zhivago would be her best overall.
Dr. Zhivago > Pure > Pirates of the Caribbean > Bend It Like Beckham > The Hole > Love Actually > King Arthur
(I'm excluding The Jacket, Princess of Thieves and anything earlier, just because I haven't seen them.) I'm just going to say that KA was not one of her better performances, simply because the movie was a bit too blockbuster-style, with little depth to the characters whatsoever.
"I like refried beans. That's why I want to try fried beans, because maybe they're just as good, and we're just wasting time." - Mitch Hedberg (1968-2005)
"Football is about if you want to run and fight for each other, if you really want to play that killer ball." - Robin van Persie, Arsenal FC