I think the American Falls at Niagara Falls is basically 100%. Nobody has ever lived going over them - at least without a barrel or something and maybe not even then............I guess the only thing that would make it less than 100 is if somebody grabs you as you hop the fence.............Somebody I knew went over the Horseshoe Falls this year after he lost his job. That's like 99.9999% I'm sure.
But, I don't know how or how long it takes you to die. If you go headfirst into a boulder, I would guess instantly. But, if not, getting banged all over while drowning, seems like the least peaceful way to die.
I've done a little research at the odds of dying going over Niagra falls are only about 25% based on the people who have actually done it, at least according to one source I found.
The drug succynylcholine is basically 100% and it is basically instant and painless, but it is very tough to get- unless you are a nurse, doctor, or maybe a pharmacist you won't be able to get it. The synthetic opiod fentanyl is basically 100% if you get enough, which isn't much, but it is a street drug so you risk prison if you try to get it, and that won't make anyone's life better. Jumping off a 200 foot high bridge is about 99%, but walking alone on the golden gate bridge will get you surrounded by cops who don't like having suicides there- there are 46 bridges in the u.s. with this height, according to wikipedia, but getting to a good spot on them must be tough, because you don't hear too much about bridge suicides in the u.s. The highest percentage mainstream method is with a gun, but this is maybe 80% if you're lucky, and if it doesn't work you can go to prison for discharging a firearm where you are not supposed to- this does happen to people, and you can get ten years or more. Some people say hanging is 70%, but there are so many variables and ways to do this so that the chances vary. Sodium nitrite is a possibility, but the percent chances are unknown; getting the antiemitic to reduce or eliminate vomiting is not easy for most people. Those are the highest percentage methods that I know of. The majority of celebrities choose hanging, based on videos from a very popular video site, and they usually have enough money so that this is not an issue. Some people say that laying in front of a train is 90%, but some people say it is lower. Wikipedia has a page called list of suicide sites which shows about 330 bridges in the u.s., all but one are near the east or west coasts, not much in the middle 0f the country here.