Apple primarily. I have also used other fruit woods (peach & cherry come to mind) with great success. The fun is in experimenting. They will all be good, just some better than others. The last cheeses I cold smoked with apple were X-sharp cheddar, chipotle cheddar, habanero cheddar, garlic cheddar (really, really good), and some bacon cheddar. I am almost out. I will have to go to the cheese factory in West Jefferson, NC for more cheese, soon.
Keep us posted on your results.
Art