I flew into Kai Tak in 1993. I was sitting in the middle berth of seats in a 747, and I could clearly see Marlboro signs on the sides of buildings flashing by out the windows before we landed.
One can kind of imagine Kowloon (the mainland part of Hong Kong) as a small peninsula surrounded by mountains enclosing it in kind of a cup like shape. Kai Tak runway 13 stuck straight out into the bay. The other end was across the road from a busy commercial district with low-rise buildings.
From spending many hours watching planes land there, planes would line up on a northeast course and descend towards a hill with a checkerboard on it, roughly aligned with the end of the runway. Before reaching the hill, they would make around a 45 degree turn to the right to bring them around to the 13 runway. This was executed within a few minutes of landing, so the planes were very low. Add this to the fact that the buildings were often taller than the approach path. Add this to the fact that the runway was surrounded by water.
I would say that Kai Tak was easily the most difficult approach in regular commerical aviation. I’m sure pilots will be around shortly to correct or confirm…
The wikipedia article seems to back me up, mostly.