I would add Vajra’s Langhe Nebbiolo. Just finished a bottle of the 2011 last night. That’s in the low $20s and delivers a real dose of nebbiolo at a very modest cost.
I’ve rarely had a bad Rhone Valley wine from a trusted producer–Cosme, Pegau, Charvin, etc. These aren’t usually over $20.
Otherwise, old school chiantis work in the $15-22 range–Frescobaldi, Monsanto, Le Bocce, even Antinori Peppoli, etc.
For whites I enjoy greek moschofileros and assyrtikos, alsace pinot gris and blanc, and some bourgogne blancs by Pernot, Fichet, and (if you can find) PYC Morey for under $30.
Of course, this presumes that these wines are stylistically what you prefer to drink.
I fail to understand how this cannot be everyone’s first question. Adam, what do you like?
Pretending you said, “whatever you like”… first reds which come to mind are a couple of Guion’s Bourgueil bottlings if you enjoy somewhat rustic Loire Cab Franc. I love it, probably more than many of my other purchases at multiples of the price, and it should probably comprise a much bigger percentage of my pile. But, you know… variety. For whites, last few years I’ve enjoyed a bunch of Collestefano’s Verdicchio di Matelica at something like $12 or 13 per. Ian D’agata wrote “This (2012) is but one of many outstanding vintages for this wine. Let me be clear: I think it is probably Italy’s single greatest white wine buy.” It’s certainly finer and more characterful than anyone could reasonably expect at this price.
Generally: like others I find tons of value in inexpensive traditional-leaning Rioja (at the lower end of pricing, yeah, Lopez de Haro flash-saled themselves from nowhere onto my “thrilled to find this when visiting the folks” radar) and also Bierzo (Mencia) reds, Chianti, Loire red/white/pink/blue/green/plaid and especially Muscadet, German Riesling…
But hey, you might loathe searing acidity or brett or anything but fruit flavors, in which case treat the above as “what not to buy”.
Also generally, if you’re bottom-feeding: flash sale sites, as long as you have the discipline to buy what you (reasonably expect to) like when the price is right, ignoring claimed “% off” etc. WTSO, Cinderella, Last Bottle - all have reliably good service, along with wildly variable inventory and deals.