Noctua NH-D14, the old behemoth, I would rather have used a Noctua NH-D15 but it is too big. Anway, it shaved -10C off from the Hyper 212 Evo both in idle and load. Good for a sustainable high overclock, only thing not really good is the fans not being PWM but I guess one cant get it all. At least they are ok silent.
A quick look inside my cramped Linux-box case: