I think we can rule out Arreat Summit and Worldstone Chamber automatically. It's really an issue of where more undead are likely to spawn, and I believe there are at least four kinds of monsters for each area in acts one through four (champions and higher can spawn in any of the types available for that area, even if not selected for normal monsters). Although D2Data.Net isn't entirely accurate, I believe the Mausoleum has two types of undead, while the Ancient Tunnels has three. Act four areas can spawn two and, with guest monsters included, act four could spawn three in the Worldstone Keep. I guess it depends on how hard you want to make it on yourself.
Sorry, I should've added a '+' to the post title, but I thought people would get the idea since the Ancient's are the only ones that can spawn there and they're static levels, anyway. As for the Mausoleum, there are archers/slingers or something else possible, but I'm going by D2Data.Net, which says Rakanishu can spawn some place other than Stony Field.
I think the best area with undead is the CS - except for melee characters :azn: . Two thirds are undead and they are tightly packed, with Diablo himself being the creamtop. It's the favourite area of my bowazon. You don't even waste time to reach it, as the River of Flame is a L85 area as well, although not as crowded.
The Mausoleum is the most boring L85 area. Everything is moving at 1-3 km/h and most of it is empty, so I never go there, not even when my characters are rather weak in the biginning of a ladder season.
The Ancient Tunnels have a random 3 out of 4 monster types and 3 of them are undead. It's not as crowded as the CS, but it's easier. The problem is that it takes a lot of time to find the Tunnels if your character cannot teleport.