Of course, Pride and Prejudice is my favorite Austen book. Fantastic story. Great characters. Emma is amusing, and another great story, but not a major preference of mine. Mr. Knightley is gallant and all, but really only works if there's some sort of projection that you can manage as a reader. I wouldn't say his character is thinly etched, but he is perhaps not fully fleshed out, as Emma is.
However. Movie-Knightley (AND Movie-Emma for that matter)? Best. Austen. Guy. Ever. Keep Mr. Darcy. Seriously, keep him. Mr. Darcy is an interesting character but only a desirable one because of the significance of the effect that Elizabeth has on him. Everyone wants someone whom they can bring out the best in, the promise that was always secretly tucked under the still waters running deep. Women are especially like that. We promote. We like the idea of seeing something in someone and being able to draw it out.
Mr. Knightley doesn't really need that.