There are methods that are close to 100%, but with any method there is usually something that can go wrong. Full suspension hanging is close to 100% if you are not interrupted in the middle of the process and if what you are hanging from holds and the knot is good. But it is very important to be in a location where you will not be interrupted, because if you are pulled down at maybe ten minutes you can be a vegetable for life.
Cyanide is said to be close to 100% if not given an antidote in time, though it is known to be very painful.
Getting to 100% is very tough, I'm not really sure if there is a way. One person survived jumping from a 60 story building, every though this would be 99%+ effective for sure.
Guns are not 100%, an average success rate of 83% in the u.s., but for people experienced with guns and knowledgeable about them they can get this % close to 100%.
Following is a list of many methods- some of them can get to 99% if you are very careful in how you go about them:
https://sanctioned-suicide.net/threads/suicide-resource-compilation.3/