5. Referral programs. 
Those are very bad in my opinion. Some projects use give very little amount of coin to airdrop registrants an if they refer someone give more coins.
These are very aggressively used to pump up telegram members.
Ex;-Current media network became the largest telegram group using 4M$ airdrop and a referral program.
I think referral program cannot be consider as very bad airdrop. Ofcourse referrer must be rewarded more because he is doing as same as bounty hunters who are advertising the projects. In the other hand though the airdrop is few, participants are not forced to participate. It's all up to individual if they take or leave it.