Necron Overlords are the toughest robots in the game. They are alot of fun to put on command barges (I like to call command barges fun boats) so they can swoop all over the board. Also overlords can really screw over your opponent. You can mind shackle scarab your opponents Chaos lord and mae the Chaos lord kill himself or use a tesseract to open worm holes and trap a trygon in a labyrinth. There is so many deliciously evil things you can do during a game with your overlord.