The greatest tapas? Well, that honor would first go to the Saptarishis, the greatest of Rishis.
Of the Saptarishis, the two who performed the longest and greatest tapas are recognized as Atri and Vashishtha.
However, in Valmiki Ramayana Atri has explicitly praised the tapas-power of his wife Anasuya as superior to his own.
And similarly, in Mahabharata and other Shastras the tapas of Vashishtha’s wife Arundhati is highly praised, such that she is said to have given discourses on spiritual matters to even all seven Saptarishis.
So as to the question of greatest tapas, I don’t think we can know for certain but I would nominate the revered Brahmarishikas Arundhati and Anasuya for that honor.
