For BEST strikes: 1878-S Obverse 1879-S 1880-S 1881-CC 1881-S 1890-S Obverse 1891-S Obverse For WEAKEST strikes: 1891-O 1892-O 1921-S
I think this is an excellent list. I fully agree with what Hobo has here. I would like to add however, and this is pretty much common knowledge among Morgan collectors...that for the most part, there are no New Orleans years that were really well struck. If you find an example that was struck early in a particular year, you will find a nicely struck example. But, for general characteristics, an "O" minted Morgan will typically exhibit a strike on the weaker side. From my experience, 1891-O and 1892-O are the worst...but none of them are known for especially nice strikes.
Rough rule of thumb I used to use was San Francisco coins were always the nicest, New Orleans ones the worst, and in the Middle Carson City ones were often better than Philadelphia. I assumed because Philadelphia often made them in much larger quantities than Carson City.