This post is dedicated in memory of Etya Sarah bat Yitzchak ha-Levi. May it be an aliyah for her neshama.
He [Rabban Yochanan ben Zakkai] used to say, "If all the sages of Israel stood on one side of a scale and Eliezar ben Hyrkanus was standing in the other side, the latter would outweigh everyone else." Abba ben Saul said in Rabban Yochanan's name, "If all the sages of Israel, together with Rabbi Eliezar ben Hyrkanus, were standing on one side of the scale and Rabbi Elazar ben Arach were on the other, the scale would tip in Rabbi Elazra ben Arach's favor."
Rabban Yochanan used to say that Eliezar ben Hyrkanus was the greatest of his students. So where did Abba ben Saul come from when he said in Rabban Yochanan's name that Rabbi Elazar ben Arach was the greatest of the five? Although there seems to be a contradiction in the mishnah, a closer look will remove all form of bafflement. These two men had specific strengths and, depending on the times, one trumped the other in his ability to contribute his talents to the community.
Eliezar ben Hyrkanus had a memory which retained, and made readily available, everything he learned. There's a well known story about the wonders of the human brain and how it manages to hold onto every experience one's environment offers it. A woman was getting brain surgery. When the scalpel touched a specific section of her brain she began singing Italian opera. Upon waking up they asked her where she had learned opera. She answered, "What are you talking about?" After several moments, though, she recalled that, as a young girl, she had gone to see an opera in Rome. Just as the human brain forgets nothing, Rabbi Hyrkanus held onto all that he learned..
In the Talmud, we call someone who knows Torah back and forth, inside out, a Sinai because he is as reliable a source of information as if he had just received the Torah at Sinai. In times of persecution i.e. the the end of the second temple period or during the pogroms in Europe, when the leaders of the Jews had been murdered or exiled to other lands so that organized learning has become nigh impossible, the need for someone who knows Torah like the back of his hand is invaluable.
Rabbi Elazar ben Arach, on the other hand, was like an ever gushing spring of water. As he aged, his enthusiasm and love for Torah did not wane. On the contrary, it knew no bounds. Thus, in times of plenty, as in, when the Torah community is chugging along, there is a demand for innovation and a mind that is always fresh. People are always looking for new viewpoints, obviously within the framework of halacha (Torah law). After all, there are seventy faces to the Torah.
Therefore, there is no contradiction between Rabban Yochanan and Abba ben Saul who speaks in his name. Before being contested, everything needs a context. Everyone has what to contribute and his moment to shine.