My personal preference is CO (with charcoal).
I presume, you won't get pure CO with charcoal, the product of burning would also include CO2 and possibly some stinky contaminations which make the method barely better than inhaling highly concentrated CO2 that may be produced in chemical reaction of sodium bicarbonate with citric acid. Inhaling >50% CO2 may knock you out very quickly.
Inert Gas / N2 (with exit bag / SCBA Mask) I'd consider it also very peaceful.
An inert gas may be used for drowning too. I don't see significant advantages of using a bag or a mask instead of water when you have an asphyxiant gas. Drowning may be even more easy and require a less amount of gas, because in this case you need an asphyxiant gas only to pass out, further deprivation of oxygen will be done with water.
Inhaling butane also needs a setup like inert gas bc butane itself isn't lethal like CO.
Butane from a gas duster may be inhaled straight from the balloon's nozzle without any additional equipment. Such balloons are more cheap and easy to obtain than cylinders with nitrogen or other inert gas.
Some drug/med ODs are certainly more peaceful than drowning.
They are much more difficult to obtain for majority of people and purity of such substances may be difficult to test. In some cases, OD and drowning may be combined in order to increase the odds of dying.
Drowning - what comes to mind: head under water, suffocation, breathing water, extreme panic, probably pain for some time. OK unless you are totally unconscious due to the use of drugs that can make you unconscious/coma so you don't feel that.
Yeah, drowning is actually not a single method, it's rather a group of methods that may imply different feelings in a wide range. Depending on the chosen strategy, you may experience unpleasant feelings, neutral feelings, or pleasurable sensations (for example, when inhaling N2O or diethyl ether) which may make drowning a luxurious method, possibly even better than death with OD of barbiturates.
I doubt that you can hold your breath until you pass out.
I was able to overcome the urge to breathe without using any auxiliary substances, but I understand that this may be difficult to do for other people.