NPC classes

I recently was rethinking the use of NPC classes in my (mostly OSR-style) games. Previously I had dismissed them as unsuccessful attempts at making overpowered PC classes. But reading them "as written" they make much more sense as human boss monsters in a lot of cases.
I specifically mean stuff like e.g. Lew Pulsipher's Necromancer class, which is a terrible class for players (it demands human sacrifices of increasing specificity to advance), but does actually make more sense when used as a maybe recurring human antagonist who keeps getting stronger as over the course of a PCs career.
I also looked at the Witch class from Dragon magazine but find that one oddly lacking.
What are your preferred NPC classes, if any?