The highest point of the world is on top of a mountain, fairly close to where you are when you finish the second agency mission and start to freeroam. Easiest way of getting there (and getting back out) is a helicopter, but take note that when you land the helicopter will probably start to slide.
If you're having trouble finding it, open your map with
. The top left corner shows coordinates. The top of the world is roughly at X:20541 and Y:11836.
While you are there, be sure to pick up the Ulah Skull, which counts for the following trophy:
Edit: sorry, I apparently mistyped on the Y coordinate, leading to only a single '1' instead of two and a very incorrect guide. Thanks to
https://www.truetrophies.com/iWantEllenPage.htm for pointing this out. I apologise for any inconvenience I may have caused with this.