The efficacy of Mantras varies with yuga. In the age of Kali (Kaliyuga), certain mantras are said to be more potent than the others:
द्वापरे शिवभेदं च त्रेतायां वामनं हरिम् |
अथ कलियुगे शस्ता या बालेति निगद्यते ||
dvāpare śivabhedaṃ ca tretāyāṃ vāmanaṃ harim |
atha kaliyuge śastā yā bāleti nigadyate ||
In Trētāyuga, the most potent mantras are of śrīmannārāyaṇa. There are various categories among Vaiṣṇava mantras such as Vaikhānasa, Pāñcarātra, Rādhāvallabha, Gōkulēśa, Vīravaiṣṇava, Rāmānandī, Harivyāsī, Nimbārka, etc. Of these, those peculiar to Pāñcarātra are said to be most potent.
In Dvāpara yuga, the most potent mantras are of Lord Mahādēva. Here again, there are various categories but the Sapta Pāśupata group is said to be the most potent (śikhī, muṇḍī, jaṭī, tridaṇḍī, etc.), among which the most potent is said to be Aghora mantrarāja (aghōrānnāparō mantraḥ).
In Kaliyuga, mantras of Bhagavatī Bālā Tripurasundarī are considered most potent for attaining siddhi. Even among the mantras of Bālā (Sundarī), recommendation varies with yuga-bheda:
त्रेतायां ललिता शस्ता द्वापरे पञ्चकूटगा |
षट्कूटा च कलौ शस्ता सर्वतन्त्रेषु गोपिता ||
tretāyāṃ lalitā śastā dvāpare pancakūṭagā |
ṣaṭkūṭā ca kalau śastā sarvatantreṣu gopitā ||
In Trētāyuga, the mantra of Lalitā i.e. Pañcadaśī (commentator Vāgbhavānanda interprets this specifically as pertaining to Hādi Pañcadaśī) is said to be siddhi-prada. In Dvāpara, Pañcakūṭā Pañcamī Mahāvidyā that emanated from the Pañcamukha Sadāśiva mūrti as the reification of Mahāmadhumatī – is said to be most suited for upāsanā. In the age of Kali, ṣaṭkūṭā Mahāṣōḍaśī vidyā (commentator here points out that this formulation is possible only with Kādi and hence the prāśastya of Kādi in Kaliyuga) is the supreme bestower of siddhi in Kaliyuga.
Also, there are certain other recommended mantras in Kaliyuga for a śākta:
बगला सर्वसिद्ध्यर्थं स्तम्भनार्थं विशेषतः |
धूमावती मारणार्थे शरभो निग्रहे मतः ||
प्रत्यङ्गिरा अग्न्यस्त्रादिनिग्रहार्थं प्रकीर्तिता |
सर्वकामप्रदा देवी सुन्दरी तु कलौ युगे ||
bagalā sarvasiddhyarthaṃ stambhanārthaṃ viśeṣataḥ |
dhūmāvatī māraṇārthe śarabho nigrahe mataḥ ||
pratyaṅgirā agnyastrādinigrahārthaṃ prakīrtitā |
sarvakāmapradā devī sundarī tu kalau yuge ||