An interesting, vivid and strong symbol from Macbeth could be the ingredients in the witches' cauldron. Using 'eye' of this and 'tail' of that has been likened to the idea of dismemberment. The reason why it would make a good symbol to use for an essay is that it is unusual. It is also relatively unexplored, so after research, you can put your own stamp on it as long as you use quotes. The historical significance is good too - the dismemberment has been likened to the way in which the country was broken up into warring factions and the faith divided into different regions/clans/families from one another resulting in religious persecution, deaths involving the 'hung,drawn and quartered' punishment,recusancy, and order/chaos.
One symbol that you could use from Shakespeare's Macbeth is the notion of disease. Macbeth's rule over his kingdom is shadowed by disease. Macbeth knows better than to commit murder yet still goes ahead with this evil act. He allows disease to take hold. The disease is not only invited into his kingdom but also into his soul and his body. The disease spreads to others under Macbeth's rule such as Lady Macbeth; the disease plagues her while she sleeps. The question now revolves around how this disease can be remedied and who can cure the kingdom of its diseased state.
Your question is not very clear. Are you looking for just any one symbol from 'Macbeth'? What is the subject of your essay?
In any case, you can cite the following lines from act2 sc.2--Macbeth's sense of guilt and fear as expressed in terms of a symbol of bloody hands:
What hands are here? ha! they pluck out mine eyes!
Will all great Neptune's ocean wash this blood
Clean from my hand? No; this my hand will rather
The multitudinous seas incarnadine,
Making the green one red.
'Hand' generally, and blood-stained hands in particular, has been a recurrent symbol in the play representing the fear-stricken, guilt-ridden psyche of the protagonist. Here in these lines, Macbeth feels appalled looking at his bloody hands bearing the traces of his heinous crime. In his visionary fear, Macbeth imagines the conversion of the universal green into one pervading red.