There's no such thing as best method, all of them will work, the PWM one is the easiest but probably not the easiest one to predict the final value. R2R is also simple, but to get really good accuracy you need at least 1% resistor. The DAC is easy but usually not the cheaper method.

"How much accuracy you need" is the keyword I guess.

Forum Rule: Please, do not double post your code all over the place, I'v already reply to that in your second thread.