If you mean where on earth does a pendulum clock run slowest, then this is likely to be at the summit of Mount Chimborazo in Equador (first climbed by the Victorian mountaineer Edward Whymper), since this is the farthest point from the centre of the earth. (Chimborazo is closer to the equator than Everest, and the Earth bulges around the equator.)

Incidentally, a certain Dr Henry Powell of Halifax was perhaps the first person to suggest that the weight of a body reduced with height.  Robert Hooke tried to check out the idea in 1664. He took a beam balance to the highest accessible place in London -- the top of the great tower of St Paul's, 62 m above the ground, which was destroyed in the Great Fire two years later. His experiment compared the weight of a lead bob suspended from a long thread attached to one pan so that the weight was near the floor far below with the weight of an identical bob and and identical length of thread entirely contained within the other pan at the top of the tower. His experiment was inconclusive -- not too surprising a result, as the weight difference would be only about 2 mg in 100g, which would have challenged even Hooke's instrumental and experimental skills. He observed that air currents and humidity variation affecting the moisture content of the thread were serious obstacles to this experiment.