Definitely airdrop because its the easiest way to earn different tokens scattered from the market. The airdrops program is the fastest way to gain a lot of tokens but only for a small quantity.
The only problem with this program is that the tokens rewarded from airdrops are often have little value. This is why i prefer bounnty campaigns because i have a huge chance of gaining great quantities of tokens though it a very long process but i believe its worth the wait.