Bestfoods, the maker of Skippy peanut butter and Hellmann's mayonnaise, agreed Tuesday to be purchased by European conglomerate Unilever for $20.3 billion, creating the largest food company in the world.The final offer of $73 a share, made nearly a month after Unilever first courted Bestfoods, was 11 percent higher than Unilever's original unsolicited bid of $18.