What an important question, and one that we as humans and as Jews return to each generation. Unfortunately, there's no truly pithy answer. There is an interesting discussion in the Talmud, Makkot 23b-24a, where the Talmud envisions various prophets as trying to boil down the Torah and its commandments into ever-smaller units. My favorite in that list, the one that I put on my father obm's tombstone, is Micah 6:8's call for doing justice, charity, and walking humbly with God. The Talmud itself goes further, ending with Amos' note that the righteous shall live by his faith. That faith, though, is an active one, one in which the believer seeks to act in the ways God wants, not just to believe.
I have actually spent awhile discussing these kinds of issues, most recently in a series of posts at blog.webyeshiva.org, called the Mission of Orthodoxy project. There are 21 so far, with about six to come after Passover, but the overall thrust is that all of Judaism is focused on guiding human beings (Jew or non-Jew-- Judaism has a very clear picture of what valuable non-Jewish life would look like as well) on how to develop themselves and their societies in Godly ways.
So that, in very brief sum, the essence of a good life is trying to get ever-closer to God, a process that involves shaping ourselves in ever more Godly ways. To know what those are, a lot of study is required, since it is not always intuitive what being Godly is like. While many times what seems intuitive is in fact what God wants, sometimes things we think of as problematic are in fact the more Godly way to act (and other times, the wrongs we dismiss lightly are in fact much more serious).
The Talmud's other candidate for a single-commandment encapsulation of the Torah is "Seek me and live," which it rejects only because that might mean that unless we fulfill all the commandments, we are not really seeking God (which is, in some sense, true, but defeats the goal of an encapsulation). So, bottom line, brief description: a "good life" is one lived in the constant search for what God wants, by studying the Scriptures God gave us, with the authoritative interpretations passed down over the generations, all of those combining to make us more Godly, which includes, at least, more kind, more concerned with others' welfare, and with trying to make a world of kindness and justice mixed together in the best possible way to usher in a time when all recognize God's Rule and try to shape their lives accordingly.
Answered by: Rabbi Gidon Rothstein