Here is my take on the T3 decks in the current format, which are GG/Magmortar/Banette.
GG - Maybe the best deck of the format, and you can add techs to help you beat both Magmortar and Banette. However, the tech against Banette does help you against mirror (Blastoise d), but hurts your consistancy, because it is need early game not later in the game, like the best tech which is Dusknoir. If you can get Dusknoir set up in Mid/Late game it can drastically help compared to Blastoise d. While it doesnt help your Mirror matchup that much, it does make the Magmortar matchup which may have been even or close to even, well into your favor, as Magmortar needs a bench, and cutting them to 4 total pokemon really can cripple them. A good Magmortar player may still be able to win, but it really does hinder them. Banette with Dusknoir is winnable if the Banette player isnt very smart, and has no experience with playing against it. I think it is well in Banette's favor depending on the build. You have to get a really good start and they have to get really unlucky for you to pull it off though. The last fact is your most likely will not event face Banette as their will not be a huge amount of players playing it, as Magmortar destroys it pretty easily, and that is a big risk depending on how many Magmortar decks are in your area. Overall I think that more than 50% of the players are going to be playing this. It maybe higher.
Magmortar - While there are many builds, and people vary on what it the best, it still needs a bench to win. I have seen some wierd Magmortar decks that have won States. However, I feel that against GG, with Dusknoir big, it will have problems, however, many players who have done well, or dont want to play GG will go with this because they feel the most comfortable. However, Dusknoir will give them problems. It does dominate Banette but can have problem with GG depending on the build. They problem with playing Magmortar is that it is very hard to go to an event and play 5-7 GG and end up winning the tournament, when players are teching against you left and right. I still expect to see this but in a lower quanity, about 25% I think will play that.
Banette - There are a few builds. It loses to Magmortar, but Magmortar can have problems with GG and that IS what makes Banette playable. GG's can play Blastoise d, but as many can tell you when you focus on setting up Blastoise d first instead of your Gardy/Gallade's you have major problems, later on. I really think that there isn't enough of this deck to be really worried about, plus Regionals or most Regionals will have 7 rounds + T16 which means a Banette Player has to go about 5-2/6-1 and can't run into any Dark or Magmortar Decks or they are done, unless they get insanely lucky. They then have to make it through 5 rounds of top cut without playing any Magmortar. I think it is too big of a risk to take. Adding Gorebyss, actually hurts consistancy, and with no way to tech against Magmortar, you may have a problem. If you can take the first few prizes you can win on time, if you take the first few prizes, if you build is solid, you may actually be able to win the game outright, if you hit the right key cards and can actually get a bit of luck. I would say about 8% of the players will actually play this. I would expect to see about 5-10 at your regionals, but I would highly doubt, unless they are being played by a good player and get some decent matchups, that you will see them in top cut.
Everything Else - While, something rogue may win, I would highly doubt it. I would expect to see about 18% of the field be this, either the fact that they don't want to play any of the above, or dont have the cards to do so. I dont know what deck you should expect to see the most of but, I think that most players are playing something other than Dark as most Dark decks lose to GG as they are weak to fighting. Honchkrow might have an arguement, but a good GG player beat's it and Magmortar should win easily.
I think that the BEST choice is GG for Regionals, however, the case can made for Magmortar, however, I expect that GG will take a majority of the Regionals as GG + Dusknoir = Win against Magmortar most of the time. I am not sure Banette will be able to make it's way through the field, but that is just an opinion.
But, who knows what really will happen when Regionals are actually here.
Drew