Aesthetics First
Judgements of good and bad are always, as far as I can tell, made on the basis of aesthetics, which are subjective but have some universality rooted in biology. Living the best life therefore can be said to mean living the most aesthetically pleasing life. Thus, there are countless possible "best" lives and no objective comparisons, just some obviously better and worse lives, as there are better and worse songs.
Lives are not meant to be a performance, but instead to be lived; therefore their aesthetic value is best judged not from the outside but from the inside.