Gambling
Signature campaign
I am earning from the above three sources since long time and pretty satisfied with the amount of bitcoins I am earning from it but sometimes gambling brings loss for me but that I recover it from trading sessions.
If earning it for free, then i can say sig campaigns are the best. But payments from signature campaign doesn't give you that much amount even if you have a high rank account. The forum restricts you to use multiple account which means you can only use one account per person. So if you have a responsibility to take care of, signature campaign payments is not enough. Imo, trading is still the best way of earning bitcoin but that will require some investments and knowledge, and soms risks also.
Signature campaigns, although do not require investments, but you have to be good in English writing to be able to earn good. Its like a writing job which not everyone can do perfectly.
Secondly, Trading is the best if you know how to trade and also if you have lots of capital to invest in trading.