I see there has been some good commentary on this topic.
Let me remind everyone that the coins that were ignored or disregarded as nearly worthless in their time gained quite some value. The problem is you have to wait for quite some time. As some have mentioned, this time is approaching as the Proof sets of the mid 1970's to early 1980's suffered often from PVC damage (you will note a haze on them) so out of necessity they need to be cleaned for PVC and stored in proper materials (Danscos are good as well as gold old coin capsules). The worst years I have seen are about 1979 - 1981 making it difficult to find a
SBA proof dollar without PVC damage.
Mint sets don't suffer as severe a degradation but as the cellophane ages the coins tend to shift within their packet or the cellophane packet s develop tiny holes. You would be surprised to see the condition of coins in 1970 Mint sets. Many intact have coins that are subpar quality as many sets have been cherrypicked for quality.
As for rare or scarce errors, Condor identified them with the small S from 1970 being the easiest to find. The No "S" ones from 1975, 1983 are quite rare and the 1990 very scarce.
One area of collecting that to me has some modest (but I don't think you will get wealthy from them) current proof and mint sets. The mintages have been under 500,000 for years matching the mintages of mid 1950's. The packaging is superior to those from the mid 1950's (I would say the turning point in improved packaging began with the 2010 Mint sets, the proof sets were already quite improve with the sets from 1992 onward. All sets are in good archival storage materials --- but note no amount of packaging prevents coins from aging , they just slow down the process tremendously. The only drawback to current sets is due to the improved packaging, their survival rate is quite high compared to those prior to the early 1980s (and of course any sets prior to 1955 have an exceedingly low survival rate as most are broken up). Still worth looking into. To me a good place to start is to try and get the Mint and silver proof sets from 2010 to 2021 to have all the
ATB Quarters. Another set to get are those from 2005 - 2009 as they feature some low mintage Sac dollars before the design was changed in 2009 as well as the special Lincoln cents. Finally, these sets (2005 - 2010) feature a satin finish which carries a small premium greater than the brilliant uncirculated finish (this is in general there are exceptions).
I myself have only gotten the 2008, 2009, 2006 and 2010 Mint sets along with the 2009 Silver Proof and 2014 Silver Proof. Starting from 2018 onward I get one silver proof set through an enrollment with the US Mint. My results? 2008 and 2009 Mint sets I got from someone from Facebook and the results were mixed, the golden dollars (Presidential and Sacs) had some spotting from the wash the Mint applies during the production process. However, the Quarters, 2009 cents and other copper-nickel coins were fine. Overall grade C-. The 2010 Mint set I got from a local coin shop and had a few to choose from, excellent quality - grade A. 2009 Proof set I got from a respected online dealer
APMEX, good quality although nothing spectacular - grade B+. 2006 Mint set got from
ebay and all looked good except for one nickel which had at its edge a reddish toning starting, probably from a hole in the cellophane - overall grade B-. The 2014 Silver Proof set I got from
ebay and overall very good except for the Sac dollar showing a spot - possibly a little copper oxidation. Overall grade B.
The best quality Mint and Proof sets received were from the US Mint. And in the case of an
American Innovation Proof set which I received only one that had a damaged coin, I was able to exchange it for a superior one.
My strategy for collecting Mint and Proof sets, I don't go out of my way to get these and after trying online, I have decided to stick with buying them from the S Mint or a very good local coin dealer with a brick and mortar shop.
In sum:
1975 - 1994 and the late 60's Mint Sets will avoid PVC damage but the coins may get an accumulation of tiny scratches (hairlines) from shifting in the cellophane pockets for decades, also any holes could cause weird toning. Still, you avoid PVC damage.
The late 1960's proof sets generally do not suffer as much as the 1975 - 1982 Proof sets from PVC damage. Any coins with haze or a greenish slime - PVC damage - try to avoid unless you get it near face value and know how to clean the PVC residue off properly. After about 1983 this becomes less and less of a problem. One aside there is one exception - the Prestige and Premiere sets from 1986 - 1992 avoid PVC damage sometimes - mine have. Remember the worst years are 1979 - 1982.
Consider looking for nice Mint and Proof sets from 2007 - Present. Mintages grow smaller each year and after 2005 remain under 500,000 for the Mint and Silver Proof sets. Clad proof sets are an option if you need to save some money just realize mintages run a little higher but again sales of these has not exceeded 750,000 in years.
Your best place to shop for these is either directly from the US Mint or a very reputable local coin shop with a large and diverse inventory of Proof and Mint sets.
I hope this helps.