The first Series from 1862 with the US Treasury Seal is still legal tender money. I think the Series of 1861 without the Treasury Seal is not usable as legal tender Money.
I`m a foreigner but I love old US Currency, therfore I will write what I think. Important for me is the $ 100 bill, because they shows the Father or better the first creator of US Currency. Mr. Benjamin Franklin. The security features he invented had existed all over the World until the 1990s. I could write again about so many US bills but today I will designate just one bill. The $20 bill Series of 1914. From this issue I have all 12 Districts and 23 different variants. The back speaks to me and let me dream. Perhaps especially too: On the back you can see a German ship and the front shows the only US president whose office hours did not follow successively. I like the portraits of Grant because he looks like a cool guy. Benjamin Harrison looks wise and gracious.