Oak for heat, oak for flavor, apple or maple when I have some. I have to pay for cherry so it is used only for special occasions. Did I mention oak?
Seriously, oak is readily available to me here and works great for heat. It will maintain a nice coal bed if you keep on top of it maintaining a properly sized fire with plenty of air. It provides a mild smoke flavor if you burn a clean fire and pay credence to the "sweet blue". I usually have some sugar maple on hand that I try to save for smoking fish, but if it sits around more than 6 months or so, it becomes "heat". as the sweet smoke flavor property it once had is gone and now it's just heat and smoke. Apple is plentiful here also and I use that for poultry. Same thing with the apple for age. Once it's completely dry, the sweetness and flavoring factor is no longer in the equation and it becomes another source of heat.
I have not been able to find a source for cherry other than purchasing bags of chips at Menards. That stuff smells very good and gives an incredible color to the meat cooked with it. Just can't justify the cost very often. I do keep a few bags on hand for when I feel like I need to use it.
I know you're question was regarding wood chunks or chips added to charcoal Gary, I just felt like typing a bunch of stuff and it landed in your thread.
Just a Minnesota stick burner's opinion. Your mileage will most certainly vary.