It depends on what your threshold for "pain" is and for how long.
I reckon the most "peaceful" way to go is via the holy grail, N. Mixed reception from people about SN, but nothing egregiously "painful" about it.
Done right, hanging will knock you out fairly quickly, which means to say that you won't really experience pain for that long.
The same could be said for a lot of things that quickly knock you out or take you out for that matter. You probably can't feel the pain if you die instantly (like firearms or swift decapitation for example).
Overdosing (non-method, by the way) is a gamble and could often be extremely painful up until you pass out, then horrendously painful after the fact if you live (which the chances are pretty high). I know from personal experience because I gamble on overdoses quite frequently. My body loves me for it /s.
Quite honestly, if there was an easy painless method available, most members of SaSu would be gone by now. Sorry to break it to you like this, but a truly painless method (in the sense you're asking for, if my thoughts are aligning properly) is kind of a fantasy.