Please note: all credit goes to Mactavish2520!
How to Obtain Top Of The World Achievement/ Trophy
This trophy requires you to stand on top of the highest point in the whole game. The point is located high in the snowy mountains so the easiest and quickest way to get there is by flying a plane or a helicopter.
The map coordinates for the highest point are X: 20541, Y: 11835. It’s a way up, so you’ll need a helicopter or plane to get up there.
On top of the mountain, an Ular Boys skull marks the summit – the point where you can attain this achievement.
Exact location of mountain on map.