Of course, with a question like this, any consideration of the most important symbols utilised by Poe in this excellent Gothic short story is going to be up for debate, so all I can do is offer you my interpretation of the most important symbols to this work as a whole. Clearly this is a tale full of symbolism from start to finish, and to understand it we need to carefully unpack the very many different forms of symbolism that are present in the tale.
Firstly, and key to understanding the tale, the act of Prince Prospero in trying to escape the Red Death and sealing himself away from the outer world with his courtiers is richly symbolic:
But the Prince Prospero was happy and dauntless and sagacious. When his dominions were half depopulated, he summoned to his presence a thousand hale and light-hearted friends from among the knights and dames of his court, and with these retired to the deep seclusion of one of his castellated abbeys... They resolved to leave means neither of ingress or egress to the sudden impulses of despair or of frenzy from within. The abbey was amply provisioned. With such precautions the courtiers might bid defiance to contagion.
Prospero's act therefore is symbolic of an attempt to cheat death--itself symbolised in the form of the Red Death. His determination to lock himself away from the troubles of the world and make merry, living life to the full, is symbolic of a figure who refuses to accept the reality and inevitability of death.
Another important symbol is the clock that appears in the black room during the masquerade ball. Note how it is described:
It was in this apartment, also, that there stood against the western wall a gigantic clock of ebony. Its pendulum swung to and fro with a dull, heavy, monotonous clang; and when the minute hand made the circuit of the face, and the hour was to be stricken, there came from the brazen lungs of the clock a sound which was clear and loud and deep and exceedingly musical, but of so peculiar a note and emphasis that, at each lapse of a n hour, the musicians of the orchestra were constrained to pause, momentarily, in their performance, to hearken to the sound; and thus the waltzers perforce cease their evolutions; and there was a brief disconcert of the whole gay company...
The clock is a symbolic reminder of the passing of time and of man's mortality, which is why the dancers' faces turn "pale" as they hear the clock chime and the merriment is forced to pause momentarily, before the revellers are able to forget this reminder of death once more and carry on enjoying themselves.
Lastly, the Red Death itself is an incredibly important symbol of death. It is only the discovery that the figure dressed as the Red Death was actually nothing more than a shadow that it is recognised that the "Red Death" was present in the castle and kills each one of the revellers. Death has won out after all, in spite of Prospero's best efforts. Death cannot be cheated.