I still think Classic wow had some of the best instances... they had a more "dungeon" like feeling.
Mists have some of the most forgettable 5 man content yet imo...
I'ave always been a huge fan of 5 man content in Wow, it's such a good way to spend time with some friends in the game, without destroying your social life.
You also rely more on you co-players, and nicely tuned instances can be a good challenge, and lots of fun.
Wotlk certainly had some of the worst ever made like Oculus, but generally i think they where ok'ish. Some of them got really annoying after several visits like The Violet Hold or Strat.
Cata was ok, but there was too much re-using old content.