Decapitation by long-drop hanging.
I understand it's a quite gruesome and violent way to go. But if you can look beyond that, it's a very quick death. It requires only a strong rope, which is very easy and cheap to acquire. As long as you have a strong anchor point, and learn to tie the right knots, probability of success is high. Once it is done, there is practically no risk of being saved/recovered and living life as a vegetable. It's also a very symbolic way to die, which further adds its appeal to me.
And the gruesome/violent part? You can save others from it by killing yourself at a quiet time or place. Or, if you are evil like me, you could use that as a weapon to traumatise as many people as possible and campaign for more peaceful ways to go such as assisted suicide provided by state healthcare. I would claim that, in the absence of convenient and peaceful alternatives, I was forced to go with decapitation by long-drop hanging.
It really is the perfect way to go in my opinion.