Technically, the worst ever is NLP (no light perception). Probably no one knows what is the worst ever that has been corrected with glasses to better than 20/200. I don't think anyone keeps records on that kind of thing. The strongest ever prescription might be up around -30, which might correspond to an uncorrected VA worse than 20/2000.
VA fractions worse than 20/400 are pretty meaningless anyway. Vision worse than 20/400 is sometimes recorded as CF (counting fingers at a certain number of feet). Eg. 20/800 is equivalent to CF10' (count fingers at 10 feet). 20/2000 is equivalent to CF 4' (count fingers at 4 feet).