I cared about the characters in The Stand more than in any book before or since. Especially Nick and Tom. Also Stu.
Maybe "caring about" the villains is not quite the right term, but I thought they were written very well. Maybe Flagg came across stronger to me, b/c I read The Stand well before he showed up in other books, and it was my first introduction to this badass. And I thought Harold was written very, very well. King just nailed the "disaffected teen." The current-day counterpart is the lonely teen that joins ISIS. Whenever the news story wonders how a teen from (good family; non-religious background; etc.) can fall as far as to join ISIS, I immediately think of Harold Lauder.