What is best in the ADF gap year army navy or airforce?

No direct experience of the others, but my son had a ball in his Navy gap year! Initial training was hard (and sometimes demoralising) but after being posted to a ship and doing training exercises in Australian waters, he had 3 months overseas in waters from PNG to Malaysia (not to mention the shore leave in exotic places). He was definitely lucky in his posting, but I think if you want the possibility of travel in your gap year, the Navy gives you the best opportunity.