I would sort the list alphabetically, so users who are aware of their code page can scroll through the list quickly. I don't think further sorting would help frankly, since I can't think of any relevant factors that you would be able to sort on. However, if the user has already gone through the process you can place their previous choices first in the list or provide a default conversion option.
There would also be users who are unaware of the code page they have used. I would consider figuring out a reliable method that a typical user could use to determine their code page and provide these instructions. Without this guidance, they may have to try dozens of code pages before moving forward.
There are some solutions that I think the user would love to have but would probably be infeasible for you to implement.
- It would be ideal if you could detect the code page instantly, the same way Google Translate detects a language.
- It would also be helpful if you could narrow down the possible code pages instead of providing the full list of 150. Instead of providing instructions as I mentioned before, you can retrieve relevant inputs from the user to help with this. Or, you can do this with an algorithm if it's possible.