Elephant island, in the Antarctic Ocean, has this one

To the Chilean naval officer who rescued the survivors of a British 1914 Antarctic expedition. It is about 500km, across the wildest seas on earth, to the nearest permanent settlements.
However, it isn’t the most remote memorial created by people.

This memorial to Astronauts and Cosmonauts was left on the moon by Apollo 15.
But this one is further still

At 25 billion km, Voyager one, although not on the planet, is our most remote memorial.
On September 19, 1989, six Lebanese terrorists boarded a DC-10 en route from Brazzaville to Paris and detonated a bomb on board as the plane flew over the Sahara Desert, killing all 155 passengers and 15 crew members on UTA Flight 772.
Eighteen years after the accident, the victims’ families returned to the crash site in huge trucks loaded with stones to build the most inaccessible memorial on the planet.

For two months they worked together under a scorching sun to draw, stone by stone, an image of the plane inside a circle 60 meters in diameter.

Around the circle were placed 170 broken mirrors, representing each of the people who died in the accident, including the terrorists.

The cardinal points mark the circle and in the north position the right wing of the plane’s tail (elevator) was raised like a plaque where the names of the victims are written.

The memorial was funded by the Lebanese government and can be seen on Google Maps at coordinates 16°51’53″N 11°57’13″E