As with best-selling products, sometimes you'll have a specific variant that outsells the others.
When that happens you should make sure to highlight that variant as much as possible. Anyplace you mention the product, that variant should be the one shown. Hero banners, product photos, email images, etc.
You could also make it the first variant shown on your store page (via reordering your options in the Shopify admin). Most themes default to the first variant that's in-stock so by reordering the variant, variant will best better placements.
These are tiny changes but if you have a decent number of products the tiny changes can start to add up.
Another thing to keep in mind is that best-selling doesn't always mean revenue or units. Some variants might not be that popular but any customers that buy them come back to become long-term customers. Think really great flavors that sound odd at first.
For those you want a deeper analysis that compares the long-term customer buying behavior to find how the variants stack up. In Repeat Customer Insights that's part of the Nth Product Analysis.
Eric Davis
Retain the best customers and leave the worst for your competitors to steal
If you're having problems with customers not coming back or defecting to competitors, Repeat Customer Insights might help uncover why that's happening.
Using its analyses you can figure out how to better target the good customers and let the bad ones go elsewhere.