Poe was a formalist in that his poetry adhered to formal meters and line measures. But he rarely wrote to any explicit verse forms. So while not free form, or free verse, his poetry is heavily structured and metrical. One of the most important things about formal poetry is not specifically writing to a template or established standardised verse form, but the ability to experiment with form and take it to the next level -- creating your own verse forms and patterns; establishing your own style and voice through how you compose that musicality. Standardised verse forms are great in that they offer a crutch or predisposed method for composition, but ultimately poets find themselves in a method that speaks most to them and that enables them best. Poets like Poe and Frost, 'though very different in theme and style, are two of my favourite poets because of how they broke formal molds and created their own scaffolds to work with. Many poets have done this, and I (not pretending to be on the same level at all) also strive toward that same goal.
Poe was a master at using the correct composition to marry with theme. Annabel Lee, for example, is perfectly timed and paced, the rhymes enrich rather than lead the narrative, and the muted words at half line measures provide a softness of voice that carries the weight of the narrators intent. The Raven is yet another example of form meeting purpose and theme perfectly. The pace quickens and bounces like a nightmarish nursery rhyme due the unnatural nature of trochaic meter; its relentless persistence and extended line measures create a breathlessness but pull the reader on in an inescapable lilt. The poem works so well on so many levels because of that. Truth is Poe's poems work because of his metrical compositions; they are constructed from the ground up in tune and time with his themes. Rather than take a verse form and force something to fit it, or write something and tweak it to fit within the constraints of a verse form, Poe created a unique metric and phonic experience for each poem. The craft in that is unmistakable genius.