It's like asking what's the fastest way of earning money. Obviously there's no sort of "best answer" to this question as it's going to be totally different for each person.
There is no fast way to earn anything, you have to grind and work hard for it. As the quote above says, it is a matter of perspective.
One can obtain bitcoin in many different ways, by mining, accepting bitcoin as payment, trading, buy and sell and many more.
Don't forget that some signature campaigns are also a way of obtaining bitcoin.