"Physics was my worst subject in school."
Translation:Fysik var mitt sämsta ämne i skolan.
Yes, but it also means just plain "worst". In this case, using sämsta has the meaning of "the subject I did the worst in", but värsta can sound like "the subject I hated the most" since it's stronger and often has less to do with the quality. Hence I would avoid it here. But they are very often synonymous, and it's mostly just a matter of nuance.