A fellow Dead Head suggested the year 1981, which I'd somehow never heard or even thought about before, and I spent some time exploring that, but while it doesn't strike me as a bad year (and in fact it exceeded my expectations), I'm still not "feeling" it.
But as I noted somewhere upthread, I have a soft spot for the year 1966, their earliest full year, because of their "innocent" garage band sound and the then-blossoming Haight-Ashbury scene. I suppose it's not as "complex" as their later stuff, but that's part of the appeal of it. It's "easy listening" Dead for an older guy like me.
But there is also 1965, the year in which they became the Grateful Dead, with an even more "innocent" garage band sound, and that's been fun to explore too. In this earliest studio recording of them (from November), they weren't even called the Grateful Dead yet, but rather the Warlocks, but it's the Dead in all but name (by which they would be called by the end of the month).
And they manage to "do what they do" in 1965 and 1966 too, it's just in places I'm less familiar with. Before they had songs like Dark Star and the Other One to jam out to, they had Caution and Viola Lee Blues (a song I used to hate but am coming around to). I may not be "hearing" their later years right now, but I'm always down for 1965 and 1966.