Who is the strongest in Hindu mythology?

 Shakti and it's manifestations are the strongest and most powerful in Hindu mythology. There are no records where any of Shakti’s manifestations were defeated or even struggled against opponents. In every encounter, Shakti triumphed with overwhelming strength and power. Unless was prevented by a celestial voice as seen in Sankachuda’s case..

One of most powerful demon was Bhandasura(who had boon from Shiva to absorb half strength of opponents). He was slained by Lalita Tripurasundari. Another is Mahishasura who defeated all gods including Brahma, Vishnu and Shiva and was killed by one of Shakti's manifestation(varies according to different texts)..

Formerly there was a notorious Dānava Mahiṣāsura with a huge body in Kṛta Yuga. He caused terror unto the Devas. He was killed by the Trident-wielding Goddess. His body was pierced with the spear.

It was this powerful Asura who, in Kṛta Yuga, defeated eleven Rudras, (twelve) Ādityas, along with the group of (forty-nine) Maruts, (eight) Vasus along with Moon-god and Sun-god. He defeated Brahmā, Viṣṇu and Maheśvara.

When the terror of the Devas was thus killed in a fierce battle, Goddess Nandā of wide eyes, after the terrible slaughter, took her bath in the Hrada (pool).