Ssshhhhh, I’m tracking half of them 
From my understanding 2003 was an odd vintage, though only having tried JJ Prum from that year at the estate. Certainly has developed some interesting secondary characteristics.
I think you have to go back further than pre-2001 to find a truly bad vintage in the MSR although some consider 1999 and 2000 poor. But in any case since 2001 I personally avoid 2003, 2006, 2009, and 2014 although there are good wines in all of those vintages. I’m also not a huge fan of 2005, so I never back fill 2005 either. Pre-2001, with good storage the 1996-98 stretch is in a zone now.
Focused (vintage and region/appellation) back-filling:
2008 Burgundy — I’ve loved this vintage for a long time; it gets mixed reviews. Perfect recipe for back-filling. 
2010 & 2015 German Riesling — until further notice, German Riesling will continue to be one of the mysteriously awesome deals in wine. The extreme nature of these vintages is a turn-off for many folks, but I love them. Another perfect recipe for back-filling.
2014 Bordeaux — perfect storm led to the best QPR pricing this region has seen since 2008, if not some time before that. The nature of the vintage is appealing to my palate, and most wines didn’t garner the hyperbolic critics praise that damn near every other vintage gets these days.
More general back-filling:
Brunello from the '90s — seems I can routinely find these for (well) under $100.
Bordeaux
German Riesling from MSR
Arcadian — because Joe continues to have a well-stocked library available for raiding
Napa and Sonoma Cabernets and Merlots — if willing to buy (relatively) “no-name” producers, lots of really charming wines to be found.
I go back further - 80s and 70s. Mature super seconds can be had for reasonable prices compared to what other regions are going for these days
Lots of good burgundies are cheaper to backfill good old vintages than buy current releases. I’m backfilling more on producer and site than vintage, but of course vintage matters. The vintages I look for are 93, 95, 96, 99, 00-02, those seem to be where I’m finding interesting wines at fair prices.
I also pick up the odd Barolo, Barbaresco, or Gattinara to backfill.
Not backfilling a lot of other wines, mostly a lot of my favorite wines just aren’t available much on the secondary market – though there are some signs this is changing. Particularly some of the recent K&L auctions have had a more diverse selection.
I have decided to stop pursuing new vintages of Red Burgundy after 2015 (and I cut WAY down on the numbers of those I bought)…mainly because of my age (65), and the ridiculous amounts of 05’s and 10’s I bought (along with some 09’s and 12’s) that should last me well into senility.
But after so many years of chasing wines, I need to do something to stay amused and in the game, so I backfill for entertainment…if I feel that the wine is a pretty good deal for its vintage, level, and producer. And, as mentioned, today’s prices for current vintages make a number of older wines seem like pretty good deals…the bargains in top vintages are rare, but the decent deals are somewhat easier to find. If I see decent prices on 99, 01, or 02 reds, I backfill…I have a fair number of those vintages left, but I feel good about adding to them. Although I should worry more about provenance in all vintages, I do worry more prior to 99…it seems like the 99’s are less fragile, but I could be mistaken. My 93’s and 96’s are dwindling, but prices seem high and again provenance is a concern, for those vintages and even more so for vintages much older. I am not enamored enough of the 95 and 98 vintages to seek them out (other than those producers that I already have in the cellar), and I was not sufficiently moved to buy 06, 07, and 08 initially nor to backfill. And I really didn’t like 11 or 13 anyway. Perhaps if I were to live long enough I might regret not buying 14’s if it turns out to be an overlooked and interesting vintage similar to 2001…but maybe not. I know there are lovely wines in some of the vintages I skipped, but one can’t have everything.
I think the issue of provenance is an important one, and although my cellar is temperature and humidity controlled, I have sold wine at auction and through retailers in years past and never once has any of them made a site visit to verify my cellar conditions. And the questionable bottles I have had over the years (not counting corked wines or premoxed WB’s), the ones that seemed less fresh or even damaged, were most often wines that I had “backfilled”. So I am very aware that I am taking that chance when I backfill.
Bad? No.
Weird? 2006, 2014.
Lots of Bordeaux - I imagine the market in the US is like it is here - dead as the dodo, so plenty of good deals at auction. Even previously successful market darlings like RMP 100 pointers from 2009 have dropped. The best antidote for any En Primeur withdrawal symptoms would be to look at current prices for previous vintages! That said, I think the market will recover in 2020, in Europe anyway.
08, 09, & 11 Langhe. I’ve enjoyed these vintages, and good deals seem to sporadically pop up. I also might live long enough for them to actually reach maturity.
The market will “recover” or at least stabilize in the U.S. because of these insane tariffs. I feel like absent the tariff there would be a major correction in U.S. Bordeaux prices, and possibly some Burgundy as well.
Last year I bought some 2000-2009 Napa cab to drink now and I also snagged a few bottles from the 90s. The new release pricing being higher than older, ready to drink now vintages makes you wonder why we chase some of the mailing lists that we do. As I get my cellar and locker to capacity, having wine that I can drink without additional aging will be more important than buying new release wine. My collection of drinkable wine is still very young although in the next 5 or so years I will have some awesome ready to drink 10+ year old cabernet. My cellar is predominantly cabernet heavy.
My young cellar is anchored by '15/'16 burg, '13 Barolo. Been trying to snag '05/'10 1er burgs at auction where I can find reasonable prices (difficult) and '04/'06/'08/'10 barolo at retail mostly (mainly UK). Would like to pick up some aged Syrah to try out, and that is probably something I will be doing opportunistically going forward. Will also backfill some Etna. Not super interested in Cali and Bordeaux, but they are crowd pleasers to have on hand for non-wine geek dinners, although maybe Syrah can step in here.
I’m not as big a fan of many 1970s vintage, plus their is increased risk due to age, storage, etc. I did just grab two 1970 Magdelaine that I am eager to try, the bottles look perfect. A ‘66 that I got from Chambers was excellent.
Oh, many 70s vintages are bad. But I’m a 79, so hope springs eternal. A recent 79 Gruaud Larose was shockingly good.
You’re right that the 80s are a far richer ground.
I did grab a 79 Duhart Milon that was damn good as well. Paid $90.
I am only backfilling missing vintages for a couple of future verticals.
As expected, there aren’t too many overlapping. Unless one is looking very specifically, it comes down to the deals presented, the diversity one has in their cellar, and ones tastes. Often this is the time of the year for distributor changes, and wickedly good close outs on back vintages do appear. When it comes to CS, I lean left bank Bordeaux, but if I come across a more forward, but quality $75 California cab being dumped for 20 bucks, I’m in.
My budget is not infinite, therefore I only back fill for actual producers and vintages I have tasted, and know to still be tasting great or still reaching their peak.
Started off my backfill goals for this year with some 2001 JJ Prum Wehlener Sonnenuhr Riesling Spatlese 