bounties are usually given to you for a job that you do, for example a translation, a wallet design, some bug fixes,... so their payments are usually a good amount. so among these the bounty is the best.

mining is also a good thing for making decent amount of money. but unlike any job, you need an initial investment in this and also it has costs such as electricity cost. and that makes it impossible for many to do it and make money.

as for airdrops it is not a valid money making method! it is a giveaway and giveaways are always small. rarely they become big enough to be worth it.