In the current "climate" that we find ourselves in, I think that CO poisoning is the best method, at least for me. Everything needed is legal to obtain, so one doesn't have to order illegal substances through shady characters and risk getting the substances confiscated, or worry about getting welfare checks after receiving the order. It's basically peaceful, as opposed to something like hanging, or drowning, or possibly even firearm, although using a firearm should be quick if done correctly, but it still doesn't seem very peaceful. May just be me, though. Inert gas is good, too, but you need to get tanks of gas, which can be a problem for some. CO poisoning pretty much just requires a small enclosure of some kind (tent), some charcoal, and a willingness to learn the ins and outs of the method so as not to screw it up, which could be very bad. Sure, an adequate meter could be useful, but they're not really cheap, and may not be an option for some. Peacefulness and reliability are the top two factors for me and CO fits that bill, at least for me.