Pre-war baseball set survey

What 5 pre-war baseball sets do you guys think are the hardest to complete. I am excluding the t206 because of the Wagner. This should be fun just to hear some different opinions on some of these classic sets.
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E107
E90
E223
N690
Good luck with any of the above the E90 should be the cheapest of all to complete.
Other sets-
Allegheny's
Just So
Alpha Engraving
Tango Eggs
Always looking for Topps Salesman Samples, pre '51 unopened packs, E90-2, E91a, N690 Kalamazoo Bats, and T204 Square Frame Ramly's
It has 640 cards in 8 series . It has the Stengel card in Series 6
and of course the Joe Jackson card in series 8.
Always looking for Chipper Jones cards.
Im a very focused collector of cards from 1909 - 2012...LOL
Do you really think the Tango eggs should be up there? Yes if the mythical Cobb and Wagner and actually proven to exist maybe.
I think there are more cards in the set than we know.
Always looking for Topps Salesman Samples, pre '51 unopened packs, E90-2, E91a, N690 Kalamazoo Bats, and T204 Square Frame Ramly's
The T206 set is easily completed if you have the $$ for the big 4.
There are dozens of sets that are basically IMPOSSIBLE to complete...
ie, N172, N403, E223, Just So, Alleghney, J=K Candy, Orange Borders, Base Ball Bats Candy, Punch, Tango Egg, Stevens Firearms, Diaz, 1921 Herpolsheimer, E271 Darby, E286 JuJu Drum, Star Player Candy, R304 Demaree, E125, E107, E123, E222, T4, T5, T204 with square frame, T209 B&W, T213-1, T213-3, T214, T215 Pirates, T216 VE, T231 (obviously), D303 Mothers, D382, 1923 Fleer, W519-2-2, Rose Postcards, etc. etc.
When new "discoveries" are found every few year, its not know what the complete checklist is on these sets... The Alleghney set started off as a complete set and was broken up, it would be tough to put back together, but I guess not really impossible.
PS. Uncataloged Tango, wow nice
Looking to BUY n332 1889 SF Hess cards and high grade cards from 19th century especially. "Once you have wrestled everything else in life is easy" Dan Gable
do you think T204 is impossible? Square frames are tough but they do come up, and there aren't that many of them. Now if you add in all the backs it might be....
The uncataloged Tango actually confirms are lot of what you've posted on the set over the years, and raises an interesting possibility of how big the set actually was- quite possibly more than double what is now known.
Always looking for Topps Salesman Samples, pre '51 unopened packs, E90-2, E91a, N690 Kalamazoo Bats, and T204 Square Frame Ramly's