As sorrows mentioned, the building has to be tall enough to die from in the fall. There are some who have landed, their legs broke and knees burst, but they survived, as cripples, or even vegetables.
You do not want to land on another human. Take your own life, not theirs.
Surface hardness doesn't matter as much. The surface could be water and you'd still die from a high enough fall, especially if you landed in a particularly fatal position.
I will add that many tall buildings simply lack access to the top, or have security blocking the top off, making many of these spots unsuitable for the task - especially when it is a building in a crowded area with many people below you. Even if you don't land on them, others seeing you splat will traumatize them.
Survivors instinct is also a consideration. Will you have the guts to throw yourself off? Or do you just want to take something to knock you out, and let yourself fall.
I suggest forgoing a building and trying a cliff-face or something. Grand Canyon is largely unguarded. So many natural points that make great spots for a death leap, surrounded by beauty, rather than the hollow visuals of man made creations. Personally I would want to die far away from humans, in a peaceful place with a beautiful view. Let animals find my corpse and feed on me. The cycle of life.