I find this claim intriguing. I'm fairly sure that whoever penned it presumed that he was saying something perfectly normative, traditional, and even unequivocal. However, while there certainly are figures in our history who would agree with this statement (such as R. Chaim of Volozhin), there are many more who would not, especially amongst Chazal.
Now, many people would immediately assume that Chazal certainly held that Torah study is the highest form of human endeavor. Chazal said that Talmud Torah k'negged kulam! However, as discussed in two posts on this topic, Talmud Torah k'negged kulam just doesn't mean that. First of all, the corresponding text in the Tosefta lists the cardinal sins of adultery, murder, and idolatry, and then says, "and lashon hara k'neged kulam," so the phrase k'neged kulam is clearly an exaggeration. Second, Chazal also say that Shabbos, Bris Milah, living in Eretz Yisrael, Tzitzis, and Gemilas Chasadim are k'negged kulam. Thus, the phrase Talmud Torah k'negged kulam does not mean that it is the highest form of human endeavor.
More relevant to our discussion is the dispute in the Talmud (Kiddushin 40b) about which is greater, study or practice. This dispute was resolved with the conclusion that study is greater. That would seem to indeed demonstrate that the highest form of human endeavor is the study of Torah.
And yet, as with so many other things, matters become more complicated when you look into it carefully. The Talmud's conclusion is not merely that study is greater. It's that study is greater because it leads to practice.
There are two very important ramifications of this statement. One is that the sort of study being discussed is the sort that leads to practice. That is not necessarily the kind of Torah study that takes place in yeshivos. For more on this, see the excellent and very important article by Rabbi Dr. Aaron Hersh Fried, "Is There a Disconnect between Torah Learning and Torah Living? And If So, How Can We Connect Them? A Focus on Middos" (link).
The second important ramification is that if study is greater because it leads to practice, then this effectively means that practice is greater! Rabbi Dr. Norman Lamm, in his excellent study of this topic, notes that many authorities interpret the Gemara to mean that study is "greater" only in the sense that it takes precedence; you have to study the Torah in order to know how to practice it:
"One could thus suggest, as indeed many have, that the assembly's preference for study is meant only in a chronological sense; it is to be propaedeutic to practice. To be sure, it is indispensable to practice and therefore has to come first, but it serves only as a means to achieve another end, namely, practice, which remains axiologically superior." (Torah Lishmah, p. 141)(It is true that Rambam held that study is indeed the highest form of human endeavor. However, the type of study that Rambam had in mind was that of philosophy, not Gemara. As the Vilna Gaon points out, Rambam was deeply affected by Greco-Islamic thought. In the role that he attributed to philosophical contemplation, Rambam represents an unusual departure from both those who preceded him and those who followed him.)
The notion that Torah study is the highest form of human endeavor is based on mystical thought, and it was created by R. Chaim of Volozhin as a response to Hassidism. Hassidism innovated new relative importance to spiritual experiences vis-a-vis Torah study, and did not require halachic expertise for spiritual leadership. R. Chaim of Volozhin responded by creating a new role for Torah study, in which it took on mystical significance beyond anything ever proposed by Chazal. (See my post Torah Lishmah and Reformations of Tradition.) Here are the words of two Rishonim who represent the normative, classical Jewish position on this topic:
“ ‘It is not the study that is the main point, but rather the practice’ – That is to say, the goal of a person’s knowledge and toil in Torah is not that he should study much Torah. The goal is nothing other than that it should bring him to practice. And that is what is written, ‘And you should study them and guard them to fulfill them’ – it comes to teach that the purpose of study is for nothing other than practice.” (Rabbeinu Bechaye Commentary to Avos 1:17; see there at length)
“It is not the study that is the main point, but rather it is a man’s good deeds that pull and bring him into the next world.” (R. Shimshon of Shantz, Comment to Sifri Acharei 9:9)Let us also consider what message we wish to send to children. Should we be "continually emphasizing" to our children that studying Torah is more important than anything else? What about being a good person - perhaps that is more important than anything else? When R. Chaim of Volozhin wrote his open letter to launch the Volozhin yeshivah, he wanted to show that learning Torah is more important than anything else, and he approvingly quoted the Kabbalist R. Chaim Vital as saying that a person who can but fails to learn Torah forfeits his share in the World-to-Come, even if he has good deeds (Lamm, Torah LiShmah, p. 139). While this does not necessarily imply that somebody who does learn Torah earns a share in the World-to-Come even if he lacks good deeds, the message that comes through is certainly that it's more important to be a Torah scholar than to be a good person. Should that be the message that we broadcast?
Right now, my community is reeling in shock over the news that an outstanding local Torah scholar turned out to be a serial predator. But you don't have to turn to such an unusual case to be disturbed about teaching that it's more important to be a Torah scholar than to be a good person. A friend of mine, who taught in a yeshivah for many years, recently mentioned to me that he knows of several yeshivah rebbeim who have deeply problematic personalities, causing great harm to students, and he observed that their behavior would not be tolerated in any job other than yeshivah rebbe. There is not necessarily any correlation between being a Torah scholar and being a good person; our community errs not only in assuming that the former is also the latter, but also in emphasizing the former over the latter. There are far too many cases of people who receive respect for their Torah scholarship but who are severely lacking in the integrity and middos department. We need to be stressing to students that more important than Torah scholarship is that you are a person of integrity and good middos.
If Chazal and classical Judaism do not maintain that the highest form of human endeavor is Torah study, then what is the highest form of human endeavor? The answer is that there isn't one. Traditional Judaism does not specify a particular form of human endeavor as being the greatest. God requires different people to do different things at different times. Keeping Shabbos, serving in the Beis HaMikdash, making a bride and groom happy, escorting the dead, learning Torah, supporting your family, building up the Land of Israel and its economy, giving tzedakah, etc., etc., all have their time and place. There is no one thing that is more important than anything else. To quote the end of Koheles: סוֹף דָּבָר הַכֹּל נִשְׁמָע אֶת הָאֱלֹהִים יְרָא וְאֶת מִצְוֹתָיו שְׁמוֹר כִּי זֶה כָּל הָאָדָם. "Fear God, and observe His commandments, for this is the entirety of man."