The adjective worst describes the thing that is the least good, like this awful pizza, the worst we've ever had in our lives.
Worst means "the least favorable outcome." You may have heard the saying "hope for the best but expect the worst." Well, it you get the worst, you know there is nothing more bad than that. If you are at your worst, you have never before felt or performed this poorly. Don't confuse worst with worse, which compares only two things. Your cold might be worse than your brother's but dad's is the worst of all.