Deciding what strategy to use is purely up to you. You can use the short description in the SD-WAN Rule configuration page to decide, or you can look at Fortinet's documentation for more examples and details.
https://docs.fortinet.com/document/fortigate/6.4.8/administration-guide/22371/best-quality-strategy
Best Quality is mainly based on the measured SLA and selecting the criteria that is most important to you.
Lowest Cost is a manual setting. You can input under SD-WAN Member config what each interface "cost" is. This could be the actual cost of the line, a cost based on price for over usage, or anything you deemed to factor into cost.
Maximum bandwidth is actually a load-balancing scheme among available links that meets SLA
