Intelligent enemies, every chance I get. After all, D&D is a world full of magical healing effects which can instantly make someone combat ready again at a moment's notice. How many times would you have to see combats play out to come to the same conclusion that virtually all of the groups I play with have come to?
- Kill the healers first
- Take out whole targets, don't mindlessly split damage
- If something is down but keeps getting back up, make sure it STAYS down.
Non-intelligent foes, or 'law abiding' foes, certainly not. Those interested in kidnapping, or otherwise hoping to negotiate from a position of power, of course not. But a group of average or above average intelligence bandits ambushing a caravan the PCs are a part of on the road, or a group of enemies in their lair which the PCs have invaded? It's incumbent on THEM to ensure the enemies can't finish off their friends, not on the enemies to fight with kid gloves on.
Mind you, in my campaign resurrection is fairly easy to come by. I don't want to needlessly destroy the time and emotional investment players have made in their characters for the sake of realistic combats. But nor do I ever want the perception or reality to be that the characters are unkillable. So my campaigns tend to be heavy on the PC deaths, often associated with gear loss (which makes it easier to give out a bunch of loot as rewards!) and very light on non-plot driven "true deaths", if that makes sense.