Scientists estimate the eye of the giant squid to be the largest eye in the animal kingdom.
Scientists estimate the eye of the giant squid as the largest eye in the animal kingdom.
Scientists estimate the eye of the giant squid to be the largest eye in the animal kingdom.
Scientists estimate the eye of the giant squid as the largest eye in the animal kingdom.
The correct idiom with the verb "estimate" involves the preposition "to be": "to estimate A to be B." This verb never takes an "as" construction.