Nothing under 10 stories for a building. Go tall. For a bridge starting at 140-150 feet but find one as high as you can. Decide if you want a solid surface to hit or water. With water you can drown after you hit the surface. Drowning is fast. It's not as bad as people say.
Google how people die from jumping. The San Francisco bridge has a lot of info on this method.
It's suggested to fall backwards if you do so you won't be as nervous. I wanted to do this, but didn't have the nerve once there. Went to two different bridges. It's not as easy as it seems. Maybe be drunk or something after you get there.
What I would do better is jump without thinking. The thinking made me stop. Looking down too much as well. A building would be easier, but they lock doors to the roof sometimes.