Breeds with large comb and wattles do best in the heat. Most Mediterranean-class chickens are a good bet for hot climates. Here are some breeds that have great heat tolerance:
Mincorca, Leghorn, Ancona, White Faced Black Spanish, Catalana.
Most other breeds of chickens are able to withstand 90 degree temperatures pretty well. Sex-Links, Wyandottes, Australorps, Plymouth Rocks, and many other common breeds do just fine. Bantams, in particular, are often heat hardy.
Stay away from the heaviest feathered breeds, like Cochins, Orpingtons, and Silkies. They do not do quite as well in the heat. Though, of course, there are always examples of people who have kept them successfully in hot weather.
