One site I've seen posting a good number of top-finishing
GT armies since 6th dropped is The Rules Lawyers:
http://www.theruleslawyers.com/2012/12/law-reviews-da-boyz-gt-2012-results-and-analysis/#more-1255
http://www.theruleslawyers.com/2012/10/law-reviews-feast-of-blades-2012-results-and-analysis-part-i/
http://www.theruleslawyers.com/2012/10/law-reviews-beakycon-gt-results-and-analysis/
http://www.theruleslawyers.com/2012/10/law-reviews-battle-for-salvation-2012-results-and-analysis-part-ii/
http://www.theruleslawyers.com/2012/10/law-reviews-battle-for-salvation-results-and-analysis-part-i/
Battleground Games & Hobbies also just had an invitational tournament for their year-long tournament series, and posted the top four finishers' lists here:
http://www.dakkadakka.com/dakkaforum/posts/list/495427.page#5149599
Two of those guys are also
GT winners (one of them multiple times).
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However in my opinion 6th is still a very wide open game. What works in a given local environment can be very much a product of the local tournament format and missions, and what armies are present locally. Almost any codex can compete now with the addition of allies, which open up much wider possibilities for list building.
One advantage Grey Knights do have is that they (unlike other
SM codices) are Allies of Convenience with Necrons, so can use them as scoring & denial units, which is nice because those are definitely two of the top codices. The third book that I regularly see scaring people (either as core list or ally) is Imperial Guard. Dark Eldar has been kicking some butt lately too, especially with Eldar allies to help with psychic defense, psychic support (farseers with Divination and/or Telepathy), and flyer defense (Fire Dragon Exarch manning a Quad gun is brutal).