Mining i've heard is not profitable anymore because of electricity costs.
Trading can be the best among them, if you have some experience and a good capital then you can start trading altcoins and earn profit in no time, just do some research and ask some advises from experts and start trading with care.
Gambling is not a suggestion for collecting bitcoins i guess, if someone wants to spend their money then gambling can be considered though.
And lending is not something for someone not too old in this industry as it involves different type of risk, only senior users should be in that field, that's what i think.
earn free bitcoin by completing tasks is good for newbies and for who have time to do tasks, these types of sites require signing up, watching videos, viewing ads etc.
I haven't went through those websites paying for watching videos which increases the websites organic traffic. There are sites paying per click in terms of dollars but I expect those sites to pay in terms of bitcoin in future.