Appian Way – Piranesi (1756)
A truly enormous version that shows its many, many details can be found at Wikipedia.

The library at Malmaison, Charles Percier and Pierre François Léonard Fontaine (1812)

Charles Robert Cockerell, The Professor’s Dream (1848)

This incredible drawing is being held hostage by the Royal Academy of Arts, which has nevertheless made some details available here. There is a much larger scan here, though it is also vastly inferior in detail.