Well, I do have a bit of an unfair advantage over the average wine drinker: my great uncles were all oenologists and California winemakers from around the turn of the 20th century. My grandfather, (whose education was classical rather than scientific and who was on the business side) was a connoisseur whose cellar was full of first quality Burgundy, Bordeaux, Piedmontese and Tuscan wines as well as California wines, from about 1900 through 1961 (limited buying after the 55's). I was lucky enough to pull a lot of corks as a youth with some very knowledgeable men (and women), including the owners and winemakers at many California wineries of the '50s and '60s, and learn from their discussions.
P.S. My absolute favorite underappreciated wine: Inglenook Charbono. I don't think it's made any longer, but the 1941 was as good as any of the '41 Cabernet Sauvignons, and that's sayign something. The '66 and '68 were legendary, the '70 was magnificent, and the '76 is probably just ready to drink. It's a pity no one I know has a bottle anymore.
I remember going into a Trader Joe's in Los Angeles in 1978 and finding three cases of '70 Inglenook Charbono for $20 a case. Wonderful wine!