Hung Gar's Gung Gee Fook Fu is the most intense, hardcore form I have encountered to date. It provides a mental as well as physical workout. Really tests your patience and will power. I haven't even known it a year but I suspect that many years from now, I will still be "learning" that set. Besides Gung Gee, I also enjoy Heart Penetrating Palm. Gives a workout similar to Gung Gee but not as long of a set. Northern Leopard is another one of my favorites. Extremely fast and powerful.
"We'll show him.....Chinese Boxing!"