Everybody wants to know the “best use” of their miles, which is sort of like wanting to know the “best use” of their money. There is no best use. But there are some better deals than others.
Most travels blogs will tell you to use your miles for first- and business-class tickets, since those provide the most $/mile “value,” but they also require hundreds of thousands of miles and lots of time searching for availability. If you’re planning a crazy honeymoon splurge, save your miles for premium seats. If you’re a normal human, use them for economy fares.