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.
There is also the concept of Pratyekabuddhas or a private Buddha. This is a being who achieves enlightenment on their own and then does not go on to spread the Dharma as Gautama Buddha did during his 50 year ministry in Northern India.
So in the sense of spreading the Dharma, this being would be seen as less great than the Buddha's proper. Certainly in texts such as the White Lotus Sutra, the Pratyekabuddhas is spoken about in lesser terms than the vast array of Buddhas and Bodhsattvas that populate that text.
There are infinite Buddha's. The 27 referenced above are the ones our Buddha got definite prophecy of Buddhahood. Though Buddhas are similar in their Morality, Mastery Over the Mind and Wisdom they do have differences arising from the length of Parami and and store of Karma.
The Buddha's who have longer Parami times generally are long lived and have loner Sasanas. Also their teaching style and and frequency of teaching changes. E.g. a Buddha who was Pannadhika during Paramis generally preach more often and while other Buddhas may through example and perhaps less frequently.
First of all no matter which version you learn as Buddhists we all believe that all of the Lord Buddhas are in the same level when it comes to understanding.there can be some differences in small things like how long they would live among humans.Being a Lord Buddha is reaching a certain level of understanding which is higher than any other being,so it is a status not a name for a person.Unlike other religions being a Lord Buddha is not out of reach for the followers,So if you for example take the time for the task as a Bodhisattva and finally become a Lord Buddha you will be equal with Gautama Buddha and every Buddha before.And the 27 you mentioned before is the number of Lord Buddhas who live in this era (Kalpa) not the amount who ever lived.So in that point of view Buddhas will be born as long as Humans exist.