I am a hardcore tournament player who has played in Rogue Trader Tournaments from Salem, Oregon, down to San Francisco, Sacramento, Los Angeles and San Diego, California. I have also played in Jacksonville, Orlando and Tampa, Florida. Right now I am playing and living in Phoenix AZ, but the tournament scene is rather dead for a city of this size, so I might need to join a league.
I try to attend as many large tournaments as I can. Although the GW Grand Tournaments get a bad rap, I enjoy them, and my goal is to win a GT one day. Also I attended Adepticon in Chicago every year and if you are a fan of 40k, this is a must because there is no other event quite like it.
In the past I would put them into 3 tiers (good, bad, and ugly) and listed the most competitive builds. But after some thought, I no longer do this, and the reasons why is that I think you can win with any codex. The codexes that I thought were in the bottom of the tiered system were Necrons and Demonhunters and I think that I can win with both of them.
I will make the argument that what everyone thinks are top tier lists are just easier to play, and not necessarily better than the other codexes. And don't get me wrong by thinking that what I am saying is that you can win with any units, and that any 1500 points is the same and any other 1500 points, but each codex has enough good units that can work together well enough to where you can build a successful winning army.
Take for example, what everyone thinks are a top tier chaos list. You have tough demon princes, plague marines that just sit around and take damage, and oblits. It is a pretty easy army to play. You can also look at what people think are top tier Eldar builds of Mech and Seer council. Mech Eldar armies work under the principle of if your opponent does not have enough anti-tank you win, if they do then you are in trouble. The seer council just has a very durable unit that just heads to the opponents, and assaults. Not much brain power required. Both those builds are super easy to play.
Necrons vs. Imperial Guard.
A lot of people think that Necrons are one of the worst armies and at the bottom if the tiers. If you look at the results of the UK GT heat #2 Necrons did well with one person coming in at 8th place (higher than any Space Marine player). Why was that possible? Because you can win with Necrons. The army plays similar to my Eldar do because they are able to shoot a lot of strength 6 shooting, and they are quite durable. Here is the problem with them, they are bad in assault, so what players have to do with them is think 2-3 turns ahead and plan accordingly. This is the same as the 4th edition IG codex. You had to plan your moves and think ahead several turns to win with this army. Now that the 5th edition IG codex is here, there is a lot less thinking that you have to do with Vendettas, Valkyries, and all of the different HS choices that the army is now on "easy mode" and requires much less thought.