There is no best. It all depends on individual and how hard you work. As for me, the best one is Google adsense. It is completely free and I earn at least $20-30 daily. Besides, there are some more where I make extra money from and they are free to join: Internet affiliate programs (clickbank), work at home data entry jobs (odesk), paid survey opportunities (url://www.surveypolice.com/, url://yellowsurveys.com/), microworkers.com.