Firearm method via the shotgun with 00 buckshot loads.
Reliability would be 10/10 especially when aimed through the mouth and as close as possible to the brainstem (or even in a near miss, extensive damage would still very, very likely result in death via bloodloss, trauma, and injuries).
Peacefulness would rank pretty low, 1/10 as it is very, very gruesome and messy for the people who witness it or find the body (blood, brain and bio matter everywhere where I ctb). However, if you are referring to whether it is painless, then I'd say 9/10 assuming that one aims correctly and actually dies instantly.
Convenience, I'd give it around 8/10 because there are places where one would not be able to use a firearm (or are more likely to be stopped before successfully following through), however, in most places as long as it is quiet and secluded, no one around, then one can safely and reliably CTB via a firearm, especially when they overcome their survival instinct and don't hesitate too much.