I think it is important to remember what Poe himself said about this incredible poem that takes us on a voyage into the psychoanalytic realms. The poem, according to Poe, explores one aspect of the dark side of human nature--"that species of despair which delights in self-torture." What is interesting about your question is that it isn't so much about what the raven symbolizes, rather what the narrator projects or puts onto the bird.
Considering the speaker therefore is essential to unlocking the meaning of the poem. We know that he is a "weak and weary" student up late studying, and that he is already drifting off to sleep when the "rapping" first starts. We also know that he is grief-stricken for "the lost Lenore" and that this grief is something he is desperately trying to overcome, but that he is clearly failing. When the raven enters therefore, it is the narrator that interprets its presence as something to do with this grief, and it is the narrator that tortures himself by asking it questions to which he knows the answer - "Nevermore" - and thus sends himself into ever-greater depths of depression. Consider the climax of the poem when the narrator says:
"Prophet!" said I, "thing of evil! - prophet still, if bird or devil!
By that Heaven that bends above us - by that God we both adore -
Tell this soul with sorrow laden if, within the distant Aidenn,
It shall clasp a sainted maiden whom the angels name Lenore--"
Note how the question sets the narrator up for another "blow" to his hope about his love, Lenore.
The raven therefore can be said to symbolize loss, despair or death, but it is important to identity how the raven suits the speaker's mournful tone and tormented projections--he is made into a symbol by the narrator himself.