I have joined so many bounty campaigns but found nothing. Only those give me some bounty which i cannot exchange. can somebody please suggest me any valid airdrop or bounty campaign which you think really worth our efforts. 
You have to identify on which bounty does have really a legit developers so they will pursue the project. If they will not work for it and they are after the sales, then expect that airdrop or bounty won't succeed.
Not only developers but also on their other infos like for example if the project is compose of legit people/teams and if their listed adress or contact numbers are correct and working properly but even if you check all of these and found out that the project is real , there might still a chance that they can be failed because of other valid reasons .
Even you found one "legit" names and addresses, once their motive is to scam people, they will scam people.