Trading is the most reliable as it mostly depends on your own skills and experience - despite market fluctuations, it depends only on you how to make best of it.
Gambling certainly our of question, mining is not profitable at the current market. Campaigns are safe, but I doubt they`re too profitable tbh.
Trading is good and it is the best source of earning all of them is good but I think the best way that we can make money is trading. Try to buy at the low price or the red market. Using bounty campaign is good as it can call as free money making you will not have to invest it is free money making source for me.