Silvers and half dimes
seem to be sleeper series. I think that 3CS are undervalued, all in the series, and business strikes after 1862 should have a higher value. I, myself, am collecting 3CS, and may go for Proofs for 1863 to 1873 when I can afford it. There really aren't that many collectors of this series. So there is opportunity going around here. Even though there aren't really many collectors of this series, 3CS Collections are pretty tough to complete. Though a short series, there are quite a few rarities, and sometimes it is hard to find 3CS that aren't scratched, holed, bent, destroyed, worn, etc.
As For Sleepers, I think some of the later dates would be good. The 1851-O Sounds like a good idea. 1859-1862 have relatively low mintages, when compared to earlier dates (Excluding the 1855).