A personal favorite, and one of the most unique Morgan Dollars in the series is the 1882-O/S.
Unused surplus dies were shipped by railroad from the San Francisco Mint back to Philadelphia at the end of 1881, and then shipped to the New Orleans Mint to be re-purposed. An "O" Mint Mark was placed over the existing "S" from San Francisco, and some of these dies were used for production in 1882, creating this unique rarity. 
This variety and the history along with it is extremely popular among collectors, as part of the "S" can be seen behind and in the middle of the "O", and is very scarce in high grade. This example earned a strict AU50 from CAC Grading and a "Strong" subgrade, noting the "S" is easier to see and more pronounced than coins with a "weak" subgrade.