I'd definitely have to say starvation. The others might be more painful at the time, but they are far faster. And many of them would have the pain stop well before you actually die, due to passing out. For starvation you will suffer days, weeks, or months, slowly dying as your life gets more and more unbearable.
Besides, it is clearly the most unworthy of a true warrior. If you're eaten, killed by loss of blood, or peeled, you were bested by your opponent. Acid, Venge, or gas are simply nature (sort of, not really but unpredictable enough to those in Kenshi's world), could be a matter of poor luck to be hit by one of those. Starvation though? Too cowardly/weak to find a beast to kill or a peasant to take it from, and haven't performed any deeds to earn the support of your people or any money. Better off just killing yourself, rather than continuing your dishonourable existence any longer, like a true Shek warrior would. ;)