Generally underscore _ works best for this, and should be viable for both OSes.

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.

A folder 1

AA folder 2

AAA folder 3

Who cares about readability and logic. My outlook work archives are a mess.

Just saying that in Nemo (or whatever the cinnamon file manager is called) you can pin a file/folder to the top through the right click menu, unless I’m remembering wrong. But I haven’t used this feature at all so I don’t know how well it works for any use case.

A dot.

Generally speaking: If you want folders to be on top, do it in your application. You should not prefix folders with “random characters” to make them listed in a specific place.

If you really want to, you could use A Foldername because A is the lowest Unicode point character that is a letter (0x41) You could also use @ (0x40). I’ve seen @Foldername in the wild a few times. I would not use numbers, because numbers are stupid, you also cannot easily change them if you want to have another folder between two already existing ones.

Some applications might ignore non-letter characters (what is interpreted as a letter, depends on your locale) on sorting, though. So the safest would be A.