Iceland.

I'll tell you why...

If we ignore the apparent slowness of time when a) attending lectures and b) queueing for something, we can use the general theory of relativity to tell us that the gravity and time are related.  Mass gives rise to gravity, and gravity affects the curvature of space-time.  So, space-time, which time is of course a part of, will be curved by the presence of a mass; more mass gives more curvature.  This curvature is experienced by an observer as 'time running slower'.  That is, if you and I had identical atomic clocks, and I sat next to a big mountain (big mass) and you flew in a hot air balloon (low mass), then when we meet up later our clocks will show that time has passed very slightly slower for me than for you.  (This effect shows up with clocks on long-duration Earth orbits, but it's a tiny, tiny amount).

But staying on the planet, time on Earth will run slowest at the position where there is the most gravity.  We should find out in better detail soon where this position actually is (http://www.newscientist.com/article/dn16784-probe-launches-to-map-earths-gravity-in-best-detail-yet.html).  It appears from the earlier GRACE data (http://www.csr.utexas.edu/grace/gallery/gravity/03_07_GRACE_earthgeoid.html) that this position is centred around Iceland.  As both these links suggest  however, the distribution of the Earth's oceans (and molten rocks etc) is not static, but changes over time and will therefore affect the point of most gravity.

In my expericene it's Slough.

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.