Asuras as you know is quite ambiguous in hinduism. Mitra and Varuna are the original Aryan Asuras. in Hinduism, they are Gods. The two sets of two brothers, and then a king and his friend are asuras to kill whom Lord Vishnu took four avataras are Hiranyaksha - Varaha, Hiranyakashipu - Nrisimha, Ravana and Kumbhakarna - Rama, Kamsa and Chanur - Krishna. But then, these were incarnations of the gate-keepers of Vishnu's heaven, in asura form, only because of the curse of Sages. After their third deliverance, they returned to their post.
Out of these Kumbhakarna was very pious, and the youngest brother of Ravana, Vibhishana, was an outright worshiper of Lord Rama. Ravana himself was a great devotee of Lord Shiva and son of a brahmin sage. Hiranyakashipu's son, Prahlad too, was a great devotee of Lord Vishnu. Similarly another great asura, Mahabali, also was a devotee of Lord Vishnu. Though he was defeated by Vamana avatara of Lord Vishnu, but he has been promised Indraship in the next creation. Some of the female asuras married mythological characters, e.g., Hidimba who married Bhim, one of the Pandava brothers. She is worshipped in Himachal Pradesh (Kulu) as a goddess. Her son, Ghatotkachha, fought and gave his life for the Pandava cause in the Mahabharata war. Lord Krishna's grandson, Aniruddha, married the daughter of Banasur, who was a great devotee of Lord Shiva.
The others are Madhu and Kaitabha which arose before Brahma could create the universe, and Lord Vishnu killed them. Two leading asuras were killed by Goddess Durga, they were Shumbha and Nishshumbha. Chand and Munda, two asuras were killed by Goddess Durga in form of Chamunda. Another one, Andhak, was a son of Lord Shiva and Parvati, and brother to Ganesha and Kartikeya. Still another, Mahishasura, was killed by Goddess Durga. Narakasura was son of Lord Vishnu and Goddess Earth.
Then the asuras in the Aryan stories who imprisoned the sun or the cows during the winter, causing a long Arctic night, are Vritra, Vaala, Namuchi, Shambara, Kuvaya, etc. Indra, fortified by Soma, finally rescues the sun.
Finally, many asuras were step-brothers to Devas, being born to the two wives of Sage Kashyapa, Aditi giving birth to Devas, and Diti giving birth to Asuras. There was a third wife of Kashyapa, Kadru. All the three were daughters of the hindu Adam according to one story, Daksha Prajapati. Kadru is supposed to be the mother of all eagles and serpents.
There are many more asuras of lesser importance but most of them are one-time minor characters. One, Maricha, who was the brother of Ravanas father-in-law, is remembered as a good person. So good and bad both.