Dungeon Keeper 2, and an honorable mention to the original Dungeon Keeper and Evil Genius.
One thing I loved about these two games (aside from the obvious appeal of playing a high-fantasy setting in reverse, as the malevolent entity the goody-two-shoes heroes are opposing) is the amount of character it had. Every creature in your dungeon is unique, and has its own needs and reactions, and since you don't control them directly, dungeon management consisted of placing rooms of the appropriate size and constructing paths so that the correct monsters had to wander by those rooms.
For instance, Bile Demons are slothful, slovenly, flatulent, perpetually hungry, and slow-moving, but they're also one of only 3 creatures in the game that can manufacture Traps in a Workshop. As a result, you want to build your Lair (where monsters sleep), Hatchery (where your monsters eat baby chicks, evil!), and Workshop close together, so that your Bile demons don't get irritated while schlepping their two-ton bulk from point A to point B.
Even more extreme are Dark Mistresses, oversexed, underdressed claw-and-magic-wielding BDSM berserkers. Most creatures dislike pain; Dark Mistresses love it, to the point where slapping them actually makes them happier. They also charge into fights after only minimal spellcasting, simply because they relish causing and suffering pain. When idle, they staff your Torture Room - which they affectionately call the "Playroom."
There's few other RTS games that give your ordinary rank-and-file units such personality, and then make it a gameplay mechanic, and I kinda love it.
