The perfect bullet is easy. It's capable of sub-MOA accuracy regardless of powder loading or barrel length. Its ballistic coefficient is sufficient that there's not much difference between shooting at the range or in a vacuum. In fact, it automatically adjusts its BC to compensate for uphill and downhill shots to insure consistent point of impact. Upon impact, it rapidly assumes the perfect mushroom shape regardless of impact velocity, retains 100% of its weight regardless of what bones it penetrates, and transfers all of its energy to the animal so the bullet drops to the ground six inches after it leaves the far side of the animal. Better yet, it would probably employ variable petals to steer its way to vital organs as it penetrates.
A fantasy, you say? Of course it is. There's no such thing as a perfect bullet. At one point in history a perfect bullet would've been a lead ball that didn't have a sprue on it from the mold. The definition of perfection will always stay a step ahead of technology.