THe Buddha said that he developed metta for seven years straight in a previous birth and as a result he became the maha-brahma and the sakka seven times. However he didnt become enlightened (Udana). However he does mention the way to become enlightened using metta- it is in conjunction with insight:
15. FROM THE ANGUTTARA NIKAYA, 4:126 (SPOKEN BY THE BUDDHA)
Here, bhikkhus, a certain person abides with his heart imbued with loving-kindness extending... over the all-encompassing world.
Now whatever therein (during that state of contemplation) exists classifiable as form, classifiable as a feeling (of pleasure, pain, or neutrality), classifiable as perception, classifiable as determinative acts, or classifiable as consciousness, such ideas he sees as impermanent, as liable to suffering, as a disease, as a cancer, as a barb, as a calamity, as an affliction, as alien, as being worn away, as void, as not-self. On the dissolution of the body, after death, he reappears (as a non-returner) in the retinue of the Gods of the Pure Abodes (where there are only those who have reached the Noble Path and where extinction of greed, hate and delusion is reached in less than seven lives without return to this world). And this kind of reappearance is not shared by ordinary men (who have not reached the Noble Eightfold Path).
Some methods are simply more efficient in giving rise to insight. IMO on a scale from the best to least likely I would place: wise contemplation (yonisomanasikara), the six sense bases, one sense base, anapanasati/walking, anything else.
with metta and insight