Modern Coins Key Dates

Standing Liberty Quarters:

1916 key
1921 semi key
1923-S semi key
1927-S semi key

Walking Liberty half dollars:

1916-S semi key
1921 key
1921-D key
1921-S semi key
1938-D semi key

Mercury dimes
1942/41
1942D/41

Yokozuna, Not sure if the detailed tail feathers Cheerios Dollar is a "key date", it's considered a pattern piece as the reverse wasn't the approved one for the strike, and not necessary for a collection.

USSID18, Not sure about the W quarters, it remains to be een if collectors consider them necessary for the "collection" of Washington quarters.
if this is the case, then I'd think those 2018 Reverse proofs from the san francisco set should count, but with a mintage of 200K, I don't think anyone would argue they should be Key dates over the 1932 D and S. it's kind of a stand alone thing, And I think this may be what happens with the W mint mark quarters.
Then there's the whole " State Quarters", " America the Beautiful Quarters", and them not being technically part of the " Washington quarters" series really.
remains to be seen how they get collected really.

That raises an interesting question. Are varieties considered key dates? I would argue no, because they're not separate dates, but rather varieties of the same date. Unless that date was already a key date, then its varieties would be too. I suppose it's all subjective. Fortcollins has a good point about condition rarities as well.

For most of us (I'm guessing) especially me, the key dates are the coins I need and don't have.

I'm not a fan of contrived scarcities, like the West Point quarters. I can't bring myself to include them on any list. JMHO.