Definitely trading , everything else is most of the time a scam and gambling make you greedy with time and you eventually lose everything If you don't want to stop. In the case of trading , If you lose there is no one to blame but your self unlike of gambling where you accuse the website of scam or cloudmining that keep closing it doors and run away with funds etc...
Yea! trading is the best source of bitcoin nowadays , but before that one should consider that he/she should put time and effort about it . 
for my experience i do some gambling to but the probability to have more bitcoin is to small