For cold winter climates, I have loved my bantam brahmas. They have chunky bodies and pea combs, so they are very cold hardy. They also seem to handle the summer heat pretty well, but it usually doesn't get above 90-95 degrees here. I just got a few bantam cochins, and they don't seem to handle the heat as well, as they seem to pant more, and appear uncomfortable on hot days. Also, the cochins have single blade combs, which is susceptible to frost bite, especially for roos, which have larger combs.
Bantam wyandottes would be another good choice. I've had full sized wyandottes and found that they handled both heat and cold quite well. This is another roundish bodied breed with a short, thick comb.