Val I know what you mean... but Chaos should be up there and I think it would be really sweet if Chaos gave GK a run for their money. It is also possible that 6th will fix some of the Grey Knight issues like 5th "fixed" Mech Eldar and Nid-zilla lists.

Chaos at least has the potential for multiple very unique builds. When IG came out everyone played them differently and it was a lot of fun; however now they are one of the most boring armies out there... A new Chaos army will be fun to play with and against at least initially, I hope it is more like the SM codex which has stood the test of time (all of 5th).