Neither one sounds natural without more context. Of these four, only one is wrong: Winter is the coldest season (X) Winter is coldest season. Winter is coldest in February.
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lucas21cy grammar book tells me that "He is tallest in his class" is wrong, and you should put "the" in front of the superlative, "tallest",I don't think that is necessary, though grammar exercise books probably want you to do that.