Here is how I would rank the various codices in terms of how powerful they are. This assumes equal scenarios, player skill and luck (something that is obviously impossible). This also assumes that each codex is using an optimized list.
Chaos Space MarinesWitch Hunters
One thing to keep in mind is that these codices are all capable of winning. As I said in "The Importance of the Army List", I firmly believe that skill, luck, scenario and matchup have a greater impact on the game. I am not really a fan of the tier system because no two armies are exactly equal in power. I like the best-to-worst list better.