Depends on how hot you're talking about and your specific riding conditions. If it's 100+, you're better off with no ventilation, as the air doesn't cool you off and just dehydrates you faster.
If you're commuting and ending up stopped frequently, fully perforated leather (front perfed at least) is great. It seems to keep the heat of the sun off pretty well and has decent airflow.
If you want the best jacket in terms of protection and ventilation, the Vason VentMax jackets are the way to go. Very popular here in Dallas. 1000 Denier Cordura for the mesh shell, leather underlays in the elbows and shoulders for extra abrasion resistance, CE armor in all spots including back. Wind goes right through it as if not wearing a jacket. If stopped at a light, though, the sun will start to cook ya. At least in the black one I wear.