The lowest heat producing card will either mean A) the smallest, cheapest, slowest card you can find, or B) a high end card that's undervolted to all hell.
For the former, you're talking about a 5450 (19MH/s @ 19W TDP), or a 7750 (150Mh/s @ 55W TDP). Both of those also have the advantage of being powered right off the PCIe slot, so you don't need to upgrade your PSU to use them.
For the latter, you can't get much better than a 7970. I got mine down to
380MH/s @ 61W. Those voltage unlocked cards can reach some awesome efficiency numbers. Not the most profitable, but the most energy efficient GPU out there.