Prophet Muhammad's achievement, as can be seen from my earlier posting, Overview On Supremacy, is nothing less than spectacular.
His achievement is impressive but not spectacular, however, if measured by the area of lands conquered. Genghis Khan and Alexander the Great achieved more. These two figures are land conquerors and empire builders, of which Muhammad is not.
Muhammad, upon him be peace, is a prophet. The measure of his greatness, therefore, has to be viewed from prophetic achievement, namely, the speed of the spread and the enduring influence of the message he brought, both in ideological (or theological) as well as practical lives. Measured from this angle, his achievement was unsurpassed by any other prophets. Even if we include the philosophers into this measure, none surpassed him.
But to compare the performance of one prophet to another is preposterous. In fact, it is forbidden in Islam. The Quran never belittles one prophet or elevates the other. The Prophet Muhammad himself considers all prophets to be his equal, except that he is their leader.
It is unIslamic, therefore, to denigrate the Prophet Noah (Nabi Nuh), saying that he is not very effective in doing his job, for in spite of spending hundreds of years, he managed only to convert a handful. His greatness lies in his perseverance, for in spite of his people refusing to listen to him, he persevered with them for hundreds of years. Any other man would have given up long ago.
Neither can we say that the Prophet Jonah (Nabi Yunus) was no good, on account that he ran away from his duty, after his people were heedless to his call. He was sent for different purpose, namely that one should not give up Allah's mercy. Those who repent will always have a second chance, as the story of the Prophet Jonah illustrates. After he repented and was delivered from the belly of the fish, he went back preaching and met with success.
Likewise with any other prophet. All of them were sent for specific time to specific people for specific reason.
Similarly, we also cannot elevate the status of the Prophet Jesus on account that he commands the largest adherents. True, the Christians are ranked as number one and made up slightly less than one third of the world population. Bigger than the Muslims who are about one quarter, or the Jews, the oldest of the Abrahamic faith, who make up less than one percent (about 0.19% according to some estimate). From Islamic perspective, the Christians are not the true followers of Jesus Christ. His true followers are Muslims. Neither are the Jews the true followers of Moses. Again his true followers are Muslims. It is a historical fact that the faith brought by Jesus had been hijacked and modified, first by St. Paul, and later by many others known as the Church Fathers.
The fact of the matter is that the achievement of the Prophet Muhammad has been decreed by Allah. In His Grand Scheme of things, all prophets combined are like a building, each filling a structure making up that building. Muhammad was the last structure that completes that building. As he puts it in his own word:
"My similitude in comparison with the other prophets before me, is that of a man who has built a house nicely and beautifully, except for a place of one brick in a corner. The people go about it and wonder at its beauty, but say: 'Would that this brick be put in its place!' So I am that brick, and I am the last of the Apostles." -Sahih Bukhari 4:735
In Allah's Grand Design, the Prophet Muhammad is the last piece that is still missing. With his coming, he seals and completes that Grand Design. It is for that reason that he is called the Seal of the Prophets.
That, however, does not reduce even an iota of Muhammad greatness. It is fair to say that while he is only one last piece, for all intents and purposes, and by any measure, it is the most important piece. For in his life, teachings and accomplishment are combined all facets of other prophets' lives, teachings and accomplishments.
For that reason, even if only his life we know, and only his teachings as per Quran and sunnah we know, whereby our knowledge on all other prophets and their teachings are oblivious to us, that would be sufficient already for us to lead our lives as Muslims. That in itself is already a proof for his merit to be the Seal of the Prophets.