Show value of default variant
You can access the default variant using {{ product.defaultVariant }}.

Because product.variants isn't an ElementCriteriaModel you can't sort it like one. You'd have to retrieve the products variants as an ElementCriteriaModel first like so:

{% set lowestPriceVariant = craft.commerce.variants({
  productId: product.id,
  order: 'variants.price'
}).first() %}

{{ lowestPriceVariant.title }}