Time Enough For Love is one of his best. Poor old Bill the mule. Need to read it after some of the other Lazarus Long books though. The Cat Who Walks Through Walls is a classic too, although more for the first half than the second.
Starship Troopers has definitely been murdered by the films. The animations were much better but still didn't go into the socio-political "yeah it sounds great but would you want to live in it ?"