For a dry, crisp hit:
* Thwack
* Snap
* Crack
* Clack
For a softer, thudding hit:
* Thump
* Thud
* Bump
For a more hollow, echoing sound:
* Whack
* Bong
* Boing
Onomatopoeia is subjective, so ultimately the best spelling depends on the specific sound you want to evoke. You can also combine different spellings to create more complex sounds. For example: "The stick thwacked against the ground, making a hollow bong sound."