In most countries it will be probably DnD. But in mine (Poland) for many years Warhammer Fantasy was THE RPG. Nowadays it is more equal. Other popular games are Call of Cthulhu and World of Darkness line. I see that recently Shadows of the Demon Lord gains popularity. There is also quite alive indie scene. We have some bigger, commercial Polish RPGs (like Monastyr, Neuroshima, Wolsung), but nowadays they are rather not played.
Honestly I couldn’t tell you. I know what I play and my friends play, but I wouldn’t even know how to find out what’s popular among strangers in my country.
On shelves at game stores D&D definitely takes up the most space. I think I’ve seen a decent amount of Pathfinder and VtM. So presumably those are selling well.