The tough part about answering your question is that there is really no actual answer. The best way to earn on youtube will depend greatly on your current capablities, interests and experience. If you are a big gamer then start streaming and recording you plaing some new games. If you are into hot bitches and rap then make some twerking videos. It might be better to put a link in the description to a CPA offer, or is might be beter to use adsense.
The truth with internet and CPA marketing is there is rarely a best way to earn, everybody has to figure out their own way but it is easier to do it based off others.