What Key Date Coin Would You All Suggest I Buy?

1921 SLQ in AG or 1927-S VG-F. Both are keys and I believe, will only go higher. The other suggestions are good too. 1914-D Lincoln in AG ;-)

coppertop5150...I beleive the great depression lasted until the late 30's and actually didn't start until the fall of 29

31S Cent is cheap based on it's low mintage because it was publicized that there would be low mintage and so people saved them. Same thing for the 31S Buffalo nickels and the 31 P, D, S dimes. Depression is why 31 dated coins were low mintage across the board. Still I think all the 31 coinage is undervalued in my opinion- low mintages and priced low- at some point these coins I think will take off. The 31S cent already had a huge increase maybe 5-10 years ago and then stablized and has been in 100-120 range for awhile now, so is poised to climb again.

Don't forget about the 1909s lincoln, you can get a nice coin for $120.
I'd say save at least $200 for the 14d so you can get a vg-f coin.
Well OK maybe not quite a fine coin, but maybe an undergraded vg.

I know I mention these before, but forgot to mention the mintage.

They only came in a commemorative set for 1994 and 1997
1994 Jefferson matte proof 167,000
1997 Jefferson matte proof 25,000

I think these coins have a good chance of going up in value,
if you get them at a good price now. I bought a 1994 set last
week for $56.00 and another last night for $40.00.
The set also come with a Jefferson commemorative dollar and
a two dollar bill. That makes getting the nickel for around $20.00.

I hope to pick up a 1997 set soon.

Also there is a 1998 Kennedy set, with a RFK dollar and a JFK half.

The 1998 Kennedy set has a matte proof JFK half dollar the key
to that set.

I think you can get the 1994 set in the $50.00 range.
The 1997 in the $150 range, and the 1998 Kennedy set I just mentioned in the $200 range.

Once I get 4 or 5 of these, I hope to send them in to be graded.

Let me know how getting a 14d lincoln at face works out for you

Oh yes, the GSA Morgans from 1882, 1883 and 1884 (BU) can be picked up for around $200 I think.

Nobody but you can decide what to buy for yourself but having a few ideas might be better than one certain coin since any of them might present themselves at any time.
I am regularly hoping to stumble across a nice 1909 S Lincoln for under $100.00 but almost every one I see has one little thing about them that I find unacceptable.
Others I like to search for are 1912 S nickels, 1938 D Halves, 1908 S Indians, 1857 half and large cents,
The 1911 S through 1915 S Lincolns are nothing to sneeze at in higher grades.
Full step 1939 D nickels come with two different reverses. While they are not all key dates, working on making up your own 1949 mint set in uncirculated might be a neat thing to do. It will cost you more than $100.00 to complete but once you get a couple of them out of the way it should be easy on the wallet. I am doing this myself.
You have the 1949 S dime and half in there and it seems all 1949 coins are a bit elusive.
Let us know what you decide and then let us see it when it gets captured.