American Silver Eagle Key Dates

To an extent, there isn't really one good answer. Unless you have a specific type of collection in mind. If you're doing a graded/slabbed collection, then there are certain condition rarities and there are also bullion sets where there are rarities due to one mint not producing a lot of bullion.
So there are a lot of variables depending on the collection.
In general, for OGP coins, there are a handful of standouts.
The 1995W proof is definitely one. It's hard to get because it was only available in the 10th Anniversary set with 4 American Gold Eagles. Production second lowest.
Lowest, marginally, was the 2019S Enhanced reverse proof. The coin was sold separately at not a bad price, but everyone and his brother tried getting it and ultimately sold out fast, within minutes.
The only variety of the series was the 2008 with the reverse of 2007 Uncirculated, that is a pretty key ASE if you are a completest.
There are a lot of sets that are hard to find, but these three I'd consider key for an OGP set.
A couple honorable mentions would be the 2011S Uncirculated for the 25th Anniversary set (only Uncirculated ASE that isn't West Point), the 2013W Enhanced Uncirculated (only ASE in that finish) and the 2016 lettered edge versions (proof and uncirculated) and the 2017S proof which was only available in the congrats set or Limited Edition Silver Proof Set.

Thanks Gilly. The 1995 W Proof is going to be the one that will always be out of my grasp.

I forgot about the 2020 V75 privy, that's there only one with a privy mark so far. Betting on more in 2026. That's a good one.

the 2019-s ERP has the lowest mintage and as time goes on, it will eclipse the 95-w. Both are must have's though.

As long as it was brought up - here is my 1995-W ASE set.

The cheapest I've seen the 1995-W ASE set online is $4,000. Most of the sets I found run between $7,000 to $8,000.

I mentioned it in my first reply.