Short answer. Your mother's, as the surviving parent. The Family Code lists consent in a specific order — father, mother, surviving parent, guardian, or person with legal charge — and a guardian's consent is only needed if there is no surviving parent at all.
What the law says
they shall, in addition to the requirements of the preceding articles, exhibit to the local civil registrar, the consent to their marriage of their father, mother, surviving parent or guardian, or persons having legal charge of them, in the order mentioned.
Family Code, Article 14 — Parental Consent for Applicants Aged 18 to 21. Read the full provision →
This applies only to applicants aged 18 to 21
Article 14 applies to a specific age bracket: contracting parties who are between eighteen and twenty-one years old, and who have not already been emancipated by a previous marriage. If that describes you, the article requires you to exhibit to the local civil registrar the written consent of a specific person, in addition to everything else the marriage license process already requires.
The order the article sets is not optional
Article 14 names five possible sources of consent — father, mother, surviving parent or guardian, or persons having legal charge of them — and it explicitly says this is in the order mentioned. That phrase matters: it is not a list of equally valid alternatives to pick from. Where your father is deceased, the next person in that sequence is your mother, who at that point is the surviving parent. A guardian only comes into play if there is no surviving parent to give consent.
How the consent has to be given
The article is specific about form as well as substance. Consent must be manifested in writing by the interested party — your mother, in this case — who personally appears before the proper local civil registrar. Alternatively, it may take the form of an affidavit made in the presence of two witnesses and attested before an official authorized by law to administer oaths. A personal appearance gets recorded directly on both marriage license applications; an affidavit instead gets attached to those applications.
Why the order matters beyond just naming your mother
The article's insistence on a fixed order — rather than simply allowing any qualifying relative to consent — means the local civil registrar is not meant to accept consent from a guardian or a person with legal charge while a surviving parent is available and willing. If your mother is alive and capable of giving consent, her consent is what Article 14 calls for; the later categories in the list only become relevant once the earlier ones are genuinely unavailable.