Is Gautama Buddha the greatest Buddha who ever existed? According to tradition, there were 27 Buddhas before Gautama Buddha.  

Is Gautama Buddha the greatest Buddha out of all the 28 Buddhas?
No! All Sammasambuddhas are equal in their attainment of Buddhahood. They are all omniscient, equal in virtue, concentration and wisdom. However the time they take to cultivate Paramitha might vary from Bodhisatva to Bodhisatva. There are three classes of Bodhisattvas, namely; Intellectual Bodhisattvas (Pannadhika), Devotional Bodhisattvas (Saddhadhika) and Energetic Bodhisattas (Viriyadhika). 

The minimum time it takes to become a Buddha after getting the confirmation from another Buddha is called "Sara Asankya Kalpa Lakshaya". Intellectual Bodhisattas take the minimum time. Our Buddha was an intellectual Bodhisatta. Saddhadhika Bodhisattas take double that time. Viriyadhika Bodhisattas take 4 times the minimum time required. Buddha Maitreya who will appear in the future is said to be a Viriyadhika Bodhisatta. But after attaining Buddhahood they are all equal.

Apart from that, there are 30 similarities and 8 dissimilarities. The dissimilarities are not directly related to Buddhahood. Ex: The tree under which they attain enlightenment, the life span, parents, wife, chief disciples, whether there's a Vinaya pitaka etc.
Read the Buddhawamsa for more details.