Back to question of most famous. When my parents ran a London hotel in the 50s, with lots of foreign guests, and I would take my guitar into the bar, I would invariably be asked for either Loch Lomond or Annie Laurie [often both], wherever in the world they came from. So I nominate those two, along with Auld Lang Syne, which, as already pointed out, is sung worldwide at least once a year.