Which character should I use in a Windows/Linux directory name to place it at the top of an alphabetically sorted list?

I use ! to sort to top, and Ω to sort to bottom. So far haven't had any compatibility problems.

For the curious: the use case for this is when you want to reduce nesting but also want a sort of "soft hierarchy" within a folder. I could separate my music folder into albums and playlists, but then I'd have a mostly empty folder, so instead I put both in the same directory and use prefix naming to sort them.