So I have done a few airdrops and the profit of them vary vastly. Here are some pointers. Disregarding the time spent the free airdrops are the safest and most profitable (usually ERC20). Be wary of the ratio ones, particularly the very well known ones (like Litecoin cash) since you might even be at a loss if you buy straight in and a don't sell in time when the airdrop completes. Do NOT send donation airdrops to receive coins, you are far better at investing in a good ICO (less risky).