Noctua NF-F12 PWM. I know not everyone likes the brown/beige color scheme. And it's not cheap, though it does have very good noise/performance characteristics for SFF.
Nidec Gentle Typhoon PWM is a solid choice. It's usually more expensive than the Noctua, but doesn't have the color scheme downsides while performing a better, but slightly louder.
Arctic F12 PWM is a nice choice and it's really similar to the Noctua in performance and noise, but has it's own color issues. Not everyone is looking for a black fan housing with white blades. It is super cheap though. Like $7.50 on Newegg. It's got bang for your buck.
I'm with psycho88 though, whatever you put in there I'm gonna replace with a Noctua. I'd rather not have this collection of cheap fans sitting in my part bins, but I do, because they're perfectly wonderful fans for giving to a friend in need or in the case of one of mine dying.