The Half-Life headcrab zombies are probably my favorite.
There are enough variations in them, that they cover both kinds of zombies, fast and slow.
There are the normal, slow headcrab zombies, which are difficult to kill with straight bullets, but, like Romero's zombies, a shot to the head puts them down for good. They are dangerous in packs, but easy to deal with by themselves, so long as they aren't throwing things at you.
There are the fast headcrab zombies, which aren't quite as resiliant, meaning that you can kill them with body shots quickly enough, but they also embody the fast kind of zombies, associated with 28 days later, and Left4Dead, which can come from anywhere, and attack you before you realize it, making them dangerous.
Then there are the poison head crab zombies, and Zombines (which are rather similar). The poison head crab zombies take a LOT of damage, are very slow, but can throw poisoned headcrabs at you from a distance, which will knock your health down to 1 immediately. The Zombines are heavily armored, and take a lot of damage, and can throw grenades at you.
Since these zombies encompass the full zombie spectrum, you never really know exactly what to expect, and dealing with them can be rather tricky, if it's your first playthrough. That's why I choose them, because you can't really get into a comfort zone with them, the way that you can with the zombies in Left4Dead, Resident Evil, or Dead Rising.
The Left4Dead zombies are a close second, because they have good variation of the Special Infected.