Philosophically, the premise is flawed. Best life... according to whom?

I mean, the best life for a slug or a fly won't cut it for you. I can imagine a fly being born in such conditions that from that fly's perspective it would be 'the best life' imaginable... for a fly.

There's this passage from Roger Crisp's Mill on Utilitarianism, where he proposes this thought experiment. There one reads:

"You are a soul in heaven waiting to be allocated a life on Earth. It is late Friday afternoon, and you watch anxiously as the supply of available lives dwindles. When your turn comes, the angel in charge offers you a choice between two lives, that of the composer Joseph Haydn and that of an oyster. Besides composing some wonderful music and influencing the evolution of the symphony, Haydn will meet with success and honour in his own lifetime, be cheerful and popular, travel and gain much enjoyment from field sports. The oyster's life is far less exciting. Though this is rather a sophisticated oyster, its life will consist only of mild sensual pleasure, rather like that experienced by humans when floating very drunk in a warm bath. When you request the life of Haydn, the angel sighs, ‘I'll never get rid of this oyster life. It's been hanging around for ages. Look, I'll offer you a special deal. Haydn will die at the age of seventy-seven. But I'll make the oyster life as long as you like...’"

So, a pig or Haydn? A fly or your own life right now?