There are quite a few key dates in the Lincoln series. A good list is as follows:
1909-S
1909-S VDB
1912-D
1912-S (I would consider the 1912s maybe as semi-keys)
1922 plain
1931-S
Any 1943 steel cent found is rare.
1955-P 1DO-001 (The famous DD)
1960-P small date
1969-S 1D0-001 (Extremely rare DD that can fetch upwards of $30,000)
1970-S small date
1982-P small date
DDR1983-P
DDR1984-P
DDO1988 RDV-006
1992 CAMs
1995-P 1DO-001
1997 DD
1998-P, 1999-P (the most valuable), 2000-P WAMs
That's about all I can think of in terms of Lincoln cents. I think any 1931-1934 cent is hard to find.