Very legible; I read it instantly, and I'm probably right about what it says.
Legible; It took a few seconds to work out, or I'm reasonably sure I have it right.
Unclear; It took me a while to read, or I'm not sure I know what it says.
Just about illegible; It took too long to read, or I'm pretty sure I have it wrong.
Illegible; I have no idea what this says.