I had cited Sunqua Vale as good low-level zone for spawn variety and placement and Fissue of Woe in the end-game. I would like to add Talus Chute in the late-game to the list of zones with good enemy selection.
A few monster types control the regions of the zone. This is enough to differentiate the areas and provide variety without becoming a mass of randomness. You also have the giants’ cave and ridge, which seems less like a ruled area than a no man’s land. I think it would have been marginally stronger to spread the ice imps more generally, less mixed with the dwarves and more of the “wildlife” of the region, and to have the nightmare pop-ups also a bit broader, making their boss spot more like a Tolkein-esque seat of corrupting evil in a natural area that has turned against the people.
The spawns are a bit dense along the frozen river, which is not an overwhelming challenge but seems like an irregular difficulty curve. You have several spawns packed together along with several patrols, although maybe it was just my bad fortune that had several patrols in synch while I was around. This also shows off the Z-axis problems in Guild Wars 1. Spells ignore vertical distance, melee cannot deal with it, and arrows are stellar from above and weak from below. If you have a party of fire elementalists, you can abuse this madly; if you are not watching the mini-map, you can fall victim to a dozen water elementalists who are blasting you through cliff faces.