While it may be true that Starswirl was known as the greatest unicorn of Equestria, he was not the most powerful.
Starswirl was a genius in magic theory but for the higher advanced spells, he needed more powerful unicorns to cast them as he did not have the sufficient magic power and control to do so.
He usually travels the world in search of new ideas and learn spells he had not yet learned. Though, only slightly more than a quarter of it had he mastered. Sometimes, he would bring in a new student he found which had the talent and power for magic.
This is why his students are known as the most powerful unicorns ever lived and while some were renowned for their greatness, others were feared due to their evil.
Not all of Starswirl’s student grew to become good ponies.