A nicely toned lincoln or a 1909 vdb. Try apachecoin.com you might be able to find a good deal. http://apachecoin.com/1909-vdbchoiceburbw9002.aspx There is a 1909 VDB for a good price. I have never ordered from them but I know the person who owns the place is a great guy.
I would purchase an early semi-key-date Lincoln in a collectible grade. The coin would be problem-free, and the dealer would guarantee the authenticity of said coin. As examples, as a collector, I would likely pay $50 for an MS-63 RED 1909-VDB, an EF 1915, a 1913-S in EF, or a 1933-D in MS-64 RED.
I'd hunt down the nicest 21-S in AU that I could find. That is one tough cookie. Much tougher than any mentioned so far.
I don't collect lincolns, and I'm not particularly a fan of uncirculated coins preferring scarcity over condition, so I would try to find someone who would sell a low grade 33s.