This should be somebody who you trust to properly administer your estate, settle debts and tax and distribute the estate according to your wishes. It is an important and sometimes burdensome task and should not be undertaken lightly.
Check with the person you intend to appoint that they are happy to act, as they could refuse to do so at a later date. It is generally recommended that you appoint close family members, as they will be best placed to know about your estate and family.
In addition, you may wish to appoint a professional (such as a solicitor or accountant), particularly if the estate and/or will is especially complex.