who has a sensible approach to life

We see Willy's wife as the most sensible of all the characters in Willy's family.  She is not fooled too often by Willy's insecurities and delusions of grandeur; she sees Biff for the failure he has become since he failed math; she understands that Happy is not nearly as important as he would like to present himself.  Linda has helped the family weather the thick and thin of their lives, so in this sense she is the most realistic.