This may seem like a disingenuous answer, but it was at its lowest in terms of power and territory when it first came into existence at the beginning of the 14th century, and also after it was defeated in World War I.

As for its height, that's more subjective. Traditionally the reign of Suleiman the Magnificent (1520-66) is seen as the empire's "golden age," a notion which is being increasingly challenged by modern historiography. But "power" is an arbitrary term and can't really be measured without a clear definition. The Ottoman state had a much greater ability to impact the lives of its population in the 19th century than it did in the 16th, for example. In terms of territory, the empire reached its peak in the Middle East in the last two decades of the sixteenth century when it conquered much of western Iran and Azerbaijan (which it lost a few decades later), and in Europe it reached its territorial peak in 1683.