I do not have the resources to do a good job myself, but I think that the Sabbath should have an entry on this list. I realize that it gets short shrift perhaps because it comes every seventh day. But I think a case can be made simply on the basis of its being the celebration of Creation, which provides an important complement to the festivals of Revelation (Pesah, Sukkot, and Shavuot) and of Redemption (Rosh Hashanah and Yom Kippor).
I also know that for a variety of reasons Yom Kippor and Hannukah are more popular in the United States, at least. But one can argue that in fact the Sabbath is really the holiest Jewish Holiday
1) it is the first holiday mentioned in the Bible, and God was the first person to observe it
2) the liturgy treats the Sabbath as a bride and queen
3) the Torah reading for the Sabbath has more parshat than Yom Kippor -- the most of any Jewish holiday
4) there is a tradition that the Messiah will come if every Jew observes the Sabbath twice in a row
How can Yom Kippor beat all that? But with all due respect to the Day of Atonement, this is certainly enough reasons to give the Sabbath her due on this page. I hope that the people who have been taking the most interest in this page can add this little bit more to it -- SR