What's the best use of our free will?
Actually we only have two ways of using our free will; 1) to serve Krishna, the Supreme Person, or 2) to serve maya, Krishna's material energy. Whichever one we choose will determine what happens next. We have this choice every second of the day, twenty-four hours a day, and seven days a week. With each choice there are millions of 'sub choices' but basically these are the two main categories.
The problem is that each choice starts a chain reaction; our next choice becomes determined by the previous one, and we have less control of what our choice will be. For example, let's say I decide not to drink alcohol. But then I make the decision to go to a party where alcohol is served. My friends are drinking, and my determination not to drink diminishes. As the party goes on and people keep encouraging me to drink, my initial decision not to drink will become more and more difficult to uphold.
When we make the choice to serve Krishna, our subsequent choices should support that first choice. Krishna will also assist in building our strength and determination to continue down that path.
So to answer your question specifically, according to Krishna the best way to use our free will is to decide to serve Krishna—to use everything in His service. When we decide that, He becomes pleased. When He's pleased, we also become pleased, and our eternal, loving relationship with Him develops.
And conversely, if we choose to serve the material energy we'll keep moving in that direction. But, since we are spiritual by nature, whatever "happiness" we get from the material energy will be temporary and ultimately end in disappointment.
I hope this is helpful.