"[T]he all important fact [on the date of valuation] is the opinion regarding the fair price of the land, which a hypothetical prudent purchaser would entertain, if he desired to purchase it for the most advantageous purpose for which it was adapted.... To arrive at the value of the land at that date, we have, as I conceive, to suppose it sold then, not by means of a forced sale, but by voluntary bargaining between the plaintiff and a purchaser, willing to trade, but neither of them so anxious to do so that he would overlook any ordinary business consideration. We must further suppose both to be perfectly acquainted with the land, and cognizant of all circumstances which might affect its value, either advantageously or prejudicially, including its situation, character, quality, proximity to conveniences or inconveniences, its surrounding features, the then present demand for land, and the likelihood, as then appearing to persons best capable of forming an opinion, of a rise or fall for what reason soever in the amount which one would otherwise be willing to fix as the value of the property."