The top 5 strongest Asuras in my opinion would be:
1. Madhu and Kaitabha
Born in the Kshirsagar from Vishnu's ear wax, this two Asuras were astronomically powerful & performed severe penance to get a boon of invincibility from Goddess Shakti. They challenged Vishnu and fought with him for 5,000 celestial years (an insane feat). Later, the Devi suggested Vishnu to use a trick to beat this demons before they could destroy all of creation. Using his genius mind, Vishnu asked for a boon from the two Asuras, they agreed, and the boon was that they will die by his hands right now. The Asuras granted the boon, then got terrified as they knew they have been tricked.
Vishnu took his Vishwaroopa form and said "even an infinite number of Asuras like you would be nothing but atoms compared to me. As the eternal protector of the universe, I shall now vanquish you for your sins." Then he used the Sudarshana Chakra & beheaded them.
2. Mahishasura
The buffalo demon who almost defeated all gods & deities, was ultimately slayed by goddess Durga in a 9 day battle.
3. Tripurasura
A trio of 3 Asura brothers, who had extraordinarily powerful boons. The might of almost all gods was needed to beat them
4. Mahabali
A benevolent and righteous demon king, he was almost unstoppable until Vishnu incarnated as Vamana to beat him. Later he was banished to the underworld
5. Raktabija
The Asura whose blood drops created his clones whenever they dropped on the ground, making him almost unbeatable until goddess Kali manifested herself and started collecting his blood drops in a bowl to prevent him from multiplying. He was then ultimately killed.