Most here are saying Ravana because they are either not aware of other Asuras or simply because Ravana is the most infamous cause of the popularity of Ramayana.......
I would say Hiranyaksha was the most powerful Asura in Hindu mythology cause of following reasons
1. He took the earth or (embodiment of earth) and hid her at the bottom of the 'Cosmic Ocean' or Primordial Ocean, this Primordial Ocean is what existed before creation of the Universe. Most other Asuras or Rakshasas were at best able to rule the earth and some defeated devas, but none is ever known to have taken earth to the bottom of Cosmic Ocean or Patala.
2. Lord Vishnu had to take birth as a Gigantic Varaha in order to Lift earth out and kill Hiranyaksha, this is the first time Lord Vishnu had to incarnate to kill a demon
3. The Varaha avtara and Hiranyaksha fought for over a 1000 years before Hiranyaksha was finally defeated. No other demon thereafter was able to fight either Vishnu or Shiva for so long.
4.If we were to take a closer look at Hindu scriptures all races become weaker with time or as time passes from Satya Yuga to Kaliyuga. Hiranyaksha was an Asura of Satya Yuga. Ravana was a very powerful Asura of Treta Yuga. Lord Rama had to fight and kill him as a human by means that were possible for humans to achieve without using his divine powers.
Other powerful Asuras were Andhakasura (son of Hiranyaksha), Hiranyakashipu( brother of Hiranyaksha), Mahishasura and Raktabeeja
