We are in Tractate Avos, chapter 2, mishnah 12, and according to some of the divisions of the mishnayos this is still a continuation of mishnah 8. The mishnah opens: "Hu hayah omer" - "he used to say" - that is, Rabban Yochanan ben Zakkai.
"Im yihyu kol chachmei Yisrael b'chaf mozna'im, v'Eliezer ben Hurkanos b'chaf sheniyah - machria es kulam":
That is, if all the Sages of Israel were placed on one side of the scale, and Rabbi Eliezer ben Hurkanos on the other side - he would outweigh them all. The idea is that there was no one in all of Israel who knew what he knew. He had an extraordinary breadth of knowledge and memory, and no one could compare to him.
This is not to be taken literally, that he knew more than all of them combined - for how could that be, and from where would he have acquired more information than all of them? Rather, it seems that the wording is somewhat rhetorical: not that he knew more than all of them together, but more than each and every one of them individually. There is no other person who could outweigh him.
The words of Abba Shaul:
His name was Shaul, and the title 'Abba' is a term of honor. He is not called 'Rabbi', and it seems he did not have semichah, though he was worthy of it - much like the Chofetz Chaim, who had no semichah and yet was worthy of the title. Similarly, throughout the Talmud we find people called 'Abba so-and-so' or 'Imma so-and-so', for a man and a woman respectively, as a mark of honor.
Abba Shaul said his words in the name of Rabban Yochanan ben Zakkai: "Im yihyu kol chachmei Yisrael b'chaf mozna'im, v'Eliezer ben Hurkanos af imahem, v'Rabbi Elazar ben Arach b'chaf sheniyah - machria es kulam". Even if Rabbi Eliezer ben Hurkanos, that 'plastered cistern', were placed together with all the Sages of Israel on one side, and Rabbi Elazar ben Arach on the other side - Rabbi Elazar ben Arach outweighs them all.
The idea here is that although it is true that no one knew as much as Rabbi Eliezer ben Hurkanos, Rabbi Elazar ben Arach was always able to add and to produce more and more, for he was a creative genius, an ever-flowing spring. Since he contained a sort of infinite store of content - the more you ask him, the more comes out of him - in a certain sense he outweighs them all.
Some understand that there is no disagreement here at all, but rather two ways of looking at the matter:
Rabbi Eliezer ben Hurkanos: the one who knows the information thoroughly, the master of breadth of knowledge.
Rabbi Elazar ben Arach: the creative genius, capable of generating more and more novel insights.
And in a certain sense, each of them is greater than the other.
Sinai and Oker Harim (the uprooter of mountains):
Others learn that we have here a dispute, one that runs more or less through the entire Shas. It is stated explicitly at the end of Berachos (64a) and also in Horayos (14a): who is preferable?
'Sinai': named for Mount Sinai - one who knows the entire Torah as it was given at Mount Sinai, as it were.
'Oker Harim' (uprooter of mountains): one who may not know everything, but is able to uproot the mountain - so sharp, brilliant and penetrating that he always has a question and always has an answer, he knows the explanation and he has something new to say.
The Gemara concludes that the 'Sinai' is the more important of the two. Breadth of knowledge is the essential thing, since everyone needs the information in order to work with it; and sharpness itself, that cleverness that manages to outwit everything, does not make its owner the most valuable person, nor even the greatest scholar. Everyone needs the raw material.
Accordingly, if we learn the Mishnah as a dispute, it turns out that Rabban Yochanan ben Zakkai is taking a side - and at the very least according to Rabbi Shimon, he ruled that the uprooter of mountains, the sharp, brilliant scholar with insight, is the greater of the two. The Tanna Kamma, on the other hand, holds that the greater of the two is the 'Sinai', the one with breadth of Torah knowledge.
In summary: As for us, all we can do is our very best in both areas at once: in accumulating Torah knowledge, which is a mitzvah in its own right, and in sharpening our thinking and our skills so as to have insight, to understand and to develop original Torah thoughts on our own.