"Had Hashem brought us to Sinai and not given us the Torah, that would have been sufficient [to be grateful for.]"
This famous line from Dayeinu has been the focus of endless divrei Torah. All of the previously mentioned items are worthy gifts, i.e. freedom from Egypt, being fed in the desert, etc. But how do we understand the "being at Sinai" having any value apart from the giving of the Torah?
The simplest answer is that "Mount Sinai" connotes the revelation of G-d to an extent that leaves no room for doubt. As Rambam (Hilchos Yesodei HaTorah 8:1) famously points out, our faith does not rest on any of the miracles that occurred in Egypt, no matter how grand, for miracles can be explained away. The revelation of Sinai, however, was an event during which all of Klal Yisroel directly experienced the Divine, and as such is the only true cornerstone of our faith. We can thus reasonably assume that the phrase, "had he brought us to Sinai" refers to Divine revelation, which is the cornerstone of our faith regardless of the Torah being given there and then as well.
This begs the question, then, why indeed are these two events combined as one? Just like the events at Mitzrayim, especially the splitting of the sea, are pillars of faith [i.e., "and they believed in Hashem and Moshe His servant"- Shemos 14:31] and are not narrated in conjunction with the giving of the Torah, so too why couldn't this ultimate moment of revelation not have been independent of the giving of the Torah? Would this not have better delineated each separate point - the belief in Hashem and the giving of the Torah?
To address this point we must understand something about the nature of the revelation. It is not a mere "proof of G-d", but rather it reveals to us how we are to grasp Hashem.
All of the names of Hashem represent different facets of Hakadosh Baruch Hu. Thus we are grasping Him as the source of kindness through those names associate with kindness; the G-d of power through names associated with that quality, and so on. But to have a correct grasp of Hashem, we must keep our perspective focused on the most central aspect of His revelation, and through it to view the entirety of His revelations.
Let us take an example of a skewed perspective. A rather primitive person views G-d as primarily the "all-powerful" and as having likes and dislikes which are communicated to us through Torah. Such a person would conclude that one should adhere to Torah only due to the expediency of falling in line with the "All Knowing and Powerful", clearly an impoverished perspective. Even if one is somewhat more sophisticated and understands G-d as also being the Source of existence, if that existence is viewed primarily as being very physical and the miztvos are but a guide to manage our world, the view in question is still a skewed one.
Hakadosh Baruch Hu therefore revealed Himself with His essence, i.e. the name of Havaya, at the moment of the giving of the Torah (see Rashi, Vayikra 24:11, that the primary revelation of Havaya was at Sinai.) Our perception of Hashem's central axis of revelation is the G-d of moral good, the G-d of Torah; all else is ancillary to the implementation and revelation thereof. Thus, the giving of Torah and the clearest revelation of Hashem are one and the same. The miracles of Egypt reveal to us an all-powerful G-d, and by implication the Creator. But only at Sinai is the "G-d of tov" revealed to us.
This understanding of the Divine fundamentally differentiates our emunah from all forms of avodah zara. Our difference with them is not only, "who is G-d?" but, more importantly, "what is G-d?" Their perception of divinity is power and might, with "desirable actions" simply as a result of that power.
Our G-d, lehavdil, is the G-d of tov, whose Torah is the expression of that tov in our world. All other attributes of G-d are simply branches and auxiliaries to this core essence.