|Ah the problems of flanking when you can't get past|
I’ve always been partial to the classic dungeon style of an underground complex full of kobolds/goblins. I feel that these dungeons usually contain a good mix of every kind of challenge. There can be traps, fights, ambushes, all kinds of skill challenges, secret passages, lighting issues, and all manner of different ways to harass the party. Sometimes in a forest, or some other dungeon locations, you cannot have all of these elements in the same dungeon.
My favourite part of the complex, though, is the challenges it presents to tactics. You have narrow passages for confined fighting, which is always tricky, though it often can give the advantage to the party. You have small rooms, cluttered rooms, big rooms, rooms with strange architecture, with such a variety which just doesn’t make sense in other locations. Having to fight in different conditions, on difficult terrain, is really good for inspiring tactics. Parties so often start an encounter saying sticking together is the best plan, but inevitably within three rounds everyone is in a different corner of the room and the wizard is surrounded. A dungeon crawl through a narrow cave complex or a crypt can force the party to learn or die. Sometimes, the party doesn’t learn fast enough, and I’m fine with that. Give a party an open playing field though, and despite being outnumbered, I find that too often they have too much rope to play with, and they’ll come up with something complicated to win easily. Keep them on their toes, and then even a group of kobolds can be a challenge, as so many parties have found out too late.