I would say the ones that push the hot air away from your computer through an exit. Xigmatek makes a great cooler I used and have since replaced for a zalman. No difference in temps ><. Both are great but xigmatek is cheaper and larger. Zalman LED is extremely low (see my album pics if you want). Certain materials also transfer heat faster than others. Take aluminum and coper for instance. Copper > aluminum so look for a heatsink that has copper contacts (both that I mentioned).