You can also earn them in Steam Events

@tewfik - if by "best" you mean quick, there is no such way, ofc it helps if to have decent artifacts but in the end you still have to grind daily/weekly; @hawkeye116477 has the correct answer.

Discord has a Mon-Fri poll that is not listed.

Refer a friend gives the most in the least amount of time, then you have Discord polls weekdays and Steam events that were already mentioned.