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.