Depends on what you have available to you. If you're in a place where firearms are easily accessible, maybe that? If not, maybe hanging or possibly drowning (location dependant). Someone had a really good idea in a previous thread about food allergies and making your suicide look like an accident, although I'm not sure it would be quick or completely painless. Jumping is also a possibility but again, it's location dependant, and I'm not sure about the success rates or how painless it is, but once you jump, you can't go back so be careful of where you go if you choose this.
Idk, I'd do some more research and go from there. I wish you the best of luck in whatever you choose, and I hope you find peace