Core i7-6700 or Xeon e3-1245 v5

[tdallen]

Agreed, the Intel stock coolers are quite efficient and purely from a cooling perspective there's no need to replace it.

 

However, the stock cooler for a Haswell Core i7 runs about 50mm tall while the Noctua I mentioned runs 37mm tall.  Here's what happens in a Q18 (the Q25b is almost identical)... The PSU is mounted from the back but visually intrudes into the center of the case from the left side (looking from the front).  The motherboard is mounted on the right so the CPU cooler intrudes into the center of the case from the right.  The bigger the PSU and the taller the CPU cooler, the closer these two are getting.  I got worried they'd starve each other for air, so I went with the Noctua to create more space between them since it is lower profile than the stock cooler.  In retrospect I was probably overly conservative so I'd recommend seeing how things will play out with the stock Skylake cooler.  If the PSU and cooler are too close for comfort, though, there are lower profile options available out there.