The coolest of all is the 600MHz Duron, at 24.5W typical. For all the numbers, here's
Sandpile.org's K7 section. In those ranks, SF means Spitfire, the code name of the 0.18-micron Duron core. TB means Thunderbird, TH is Thoroughbred, PM is Palomino, MG is Morgan (newer Durons), and BT is Barton.
edit: the ones with no designator are the SlotA Athlons.