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What are the key date clad quarters? I've heard 1969 and 1983, what others are there?
It really depends on condition because what might be difficult in VF or better might be easy in VG. What's easy in full strike can be difficult without scratches.
The most widely collected condition among specialists and advanced collectors is nice well made coins in chBU (here defined as MS-64 or better). Some series don't have such coins available widely so advanced collectors settle mostly at a little lower frade like attractive AU.
None of the regular issue clad quarters is scarce or rare in chBU but all are scarcer than is believed and several are far scarcer than most imagine. This is weighted a little to nice strikes but I'd rate them as '69, '82-P, '83-P, '71, '83-D, '82-D, then '77. Many of the varieties are actually rare in any condition or will be by the time people start setting them aside. Take for instance the '72-D type b reverse. This coin is "common" with a mintage around 80,000 but virtually every single specimen went into circulation so it's rare above XF. Over the years 35,000 of these have been destroyed in floods and fires or lost. The survivors are disappearing at the rate of 3% each year. Each year the survivors are more heavily worn and more likely to become culls yet still only a few collectors are looking for them.
In higher grades then the '83-P is king of the hill with nice clean surfaces but well made '82-P's that are clean are even scarcer. Other dates emerge as tough in high Gem grades because the mint set coins can be scratched or have other problems. The late-'80's and early-'90's issues come very PL but often marred by scratches. Something like the '92-D comes Proofy and scratched or it comes Gemmy but it is very tough in Gem. The '81-P is quite elusive with nice pleasing and attractive surfaces. It comes markfree but will have wavy and "porous" surfaces.
Even the '72-D which is the nicest of all clad quarters doesn't exist with a 100% strike because the bottom of the "2" is always flat. For this reason I'd say the highest grade for this date is really MS-67 but it's "common" in MS-67. There are lots of dates that don't attract much attention but really deserve some. The '72-P is often a very beautiful coin in the mint set. About 3% of mint set coins are extremely well made and very Gemmy. But roll coins of this date are scarce and are usually baggy. While there are lots of Gems for the date there are not a lot of nice attractive coins. Only about 12% of the '72-P's are nice attractive coins. The '73-D is tough by the roll and mint set coins tend to have marks. It will be tough in MS-64 but is about typical as far as its incidence in Gem. Nice Uncs aren't too tough. The '66 is overlooked as far as just hole filling Uncs are concerned. By this I mean non-SMS decent Uncs (Ms-61 & MS-62). Most of these look like junk and it's not an extremely common date. Four or five million were set aside but most of these no longer exist.
Each date has its own "sweet spot" for collecting. This is the grade right before the price jump. While I'm not in favor of investing the sweet spot for investing is right above the sweet spot for collecting. The recognized rarities, though will be the dates that are tough in nice condition. This will certainly include clean '69 coins and well made '83-P's.
Time don't fly, it bounds and leaps.