Assembly seems to be the language of choice. But for those looking for something more portable, WDC provides a C-compiler. It's not necessarily more easy to use than assembly as Foenix programming is very close to the metal and it has some quirks. But if you have any skill using C on other platforms it will make you feel right at home and it does not require you to learn 65816 specific opcodes. Good assembly will always be the better performer however. You can even mix both if you want to.
For some pointers on using the compile see C-compiler use