I think even before any serious research, once I became committed to ctb'ing what came to mind was some sort of poisoning. I would only consider resorting to something else if the poisoning option was somehow off the table.
My chosen method, before finding SS, was cyanide. But here the abundant amount of comments assuring that it's a painful way to go made me reconsider. So I went for my second option, carbon monoxide (CO) produced by chemical reaction. Further research corroborated that CO is a good painless and quick method.
Recently however I've been seriously considering a switch to hydrogen sulfide (H2S) gas because, when compared to CO it requires 10 times less concentration in the air to be certainly lethal.
The H2S method is also called "detergent suicide", because it is that method in which you can produce the lethal gas by mixing two common household products from a list. I however won't go that route, I prefer to mix pure chemicals. I'm not in a hurry to ctb right now, so I'll be conducting experiments in the following weeks to see if I can really properly pull off this method.