Pentabarbitol, fentanyl, a bullet through the brainstem, being blown apart at the atomic level by a bomb, inert gas or carbon monoxide. These have all been demonstrated to be extremely painless (perhaps some dizziness) whereas sodium nitrite does have headache and risk of nausea and vomiting. Even partial hanging can knock you out with no pain if done correctly.
So, in my opinion, no SN is far from the least painful method. There are some that have used pain medications in anti-emetics to control the headache and nausea and have gone very peacefully with SN, but that is not guaranteed.