If you work for a company that partakes in the tradition of annual self-reviews, remember to always rate yourself highly. Preferably the highest rating possible.
Why? It's not your job. It's not your job to tell your employer that you aren't meeting expectations. That's your supervisor's job.
If you didn't meet expectations, it's because you weren't told what those expectations were or weren't told what, exactly, to improve or how to improve.
Give yourself the highest rating. Because at the end of the day, it's just a job, and they would drop you the second they felt you were no longer needed.
They are perpetually wielding the hatchet, threatening workers with it. You don't need to give them a single additional reason to drop that hatchet on you.