“best” greatly depends on context.
guid is probably (with some debate about it) the best way to uniquely identify a post, but not all import/export plugins leave it alone without changes.
ID is a good identifier if you do not care about import and export.
Slug is problematic because they might be changed by the user.
The best thing to do if you need to know which post you should treat in a special way is to have an admin configuration for it that saves its ID. Then when you need to work with the same code in different environments you can just change the configuration without changing the code.