SN is described not so much as painful but rather more uncomfortable, and most of SN's side effects can be made more manageable with pain killers and properly timed benzos.
The Inert Gas method is also said to be very peaceful, either Nitrogen or Helium (getting high purity Helium is very difficult though). It can also be done with Argon, but I've heard a lot of failed attempts with Argon so I can't recommend using Argon for the Inert Gas method.
With the proper dose, barbiturate and opioid overdoses are also rather peaceful methods.
None of these are 100% foolproof unless if you can somehow manage to get a professional to do it for you. But if done correctly should absolutely make you CTB. Out of the 3 admittedly I do think the barbiturate/opioid overdose is the least reliable. You have to make sure you get the correct dosage in relation to your body mass, previous tolerance built up and any digestion issues you may have, along with antiemetics to make sure you don't throw up.
Difficult to say which is more reliable between SN and Inert Gas method, just comes down the preference really.
Drowning can be very reliable and shouldn't be too easy to mess up if you get an anchor to keep yourself submerged... But I wouldn't imagine drowning to be 'peaceful' in any way, especially compared to the previous 3 methods I've mentioned. Jumping's better than trying to get run over, that's all I can say. But even at incredibly high heights (100+ meters), there's sadly still a very small chance of surviving. People have managed to survive falling out of planes before.