It's hard. I know a great many excellent portrayals of the undead. But if there's one that immediately springs to mind when I hear the word, it would be the dead from Garth Nix's Old Kingdom series (the books Sabriel, Lirael, Abhorsen). Particularly in Sabriel.
Basically, the story takes place on a continent split in half by a giant stone wall. South of the Wall is an area that seems vaguely Great War period-esque called Ancelstierre. North of the Wall is the Old Kingdom, a feudal territory where magic permeates everything and causes technology to fail.
The undead themselves are an interesting mix. They go from Hands, basically just zombies animated with necromancy. Then you have Shadow Hands, incorporeal creatures able to harm the living but virtually immune to retaliation (even most forms of magic). There are strange creatures made from mud and blood that can cross back and forth between Death and the material world. And finally there are the Greater Dead, fully sentient monsters with powerful necromancy.
But it's the people of Ancelstierre that I feel for. When the wind blows from the north, magic crosses the Wall and plays havoc with technology in the area just to the south. Then an army built for trench warfare, managed by bureaucrats far to the south who don't appreciate the nature of the conflict, finds itself facing dead foes just as the weapons decide to fail.
To quote a soldier from the Perimeter, "This crossing point has seen too many battles, too many dead. Before those idiots down south took things under central command the crossing point was moved every ten years, up to the next gate on the Wall. But forty years ago some... bureaucrat... decreed that there would be no movement. It was a waste of public money. This was, and is to be, the only crossing point. Never mind that, over time, there would be such a concentration of death, mixed with Free Magic, leaking over the Wall that everything would... not stay dead. When I arrived the trouble was just beginning. Corpses wouldn't stay buried - our people or the Old Kingdom creatures. Soldiers killed the day before would turn up on parade. Creatures prevented from crossing would rise up and do more damage than they did when they were alive."
It's nasty, nasty stuff being posted to the Wall. Mass bouts of "mass hysteria" led to traumatised soldiers being rotated off the posting in droves. The worse the situation is, the more likely it is that the technology you count on won't work. And all the while your commanders are sending you more ammunition and barbed wire to use against wraiths and zombies that barely notice either of them.
