The type of poem that was the most popular during the mid-eighteenth century, at the height of the Neoclassical era was a heroic epic. So, the era is Neoclassicism, which means that it is a "New Classicism," when authors tried to imitate the art which was created during the Classical era. The most popular type of poems at that time were heroic epics, so if Neoclassicism finds inspiration in Classicism, obviously the most popular type of poem then is going to be the same thing.