In my opinion besides math.SE the most important other site would be [Academia][1], however this being impossible for the time being as explained by Anna Lear, my next three top-choices (I limit myself to three as there seems to be some concern regarding having too many open) would be (somewhat in order of relevance):

1. [TeX][2]

1. [Mathematica][3]

1. [Cross Validated][4]

I agree with other contributors, especially Asaf Karagila and Neil Strickland, that there seems little reason to wait; there is enough historical evidence to know that such migrations are relevant. I do not see much reason this issue dissapears now just since we changed into the network (though it might help). 

In any case, I think at least the path to TeX should be open as soon as possible; I think most instances there are also uncontroversial. This also applies mainly to Mathmatica, I think. For statistics questions there is more of a potential risk for controversy if a question or other should be migrated. But at the very least for TeX I would consider it as unfortunate if we would not get the paths now that it is possible; that now *one could migrate questions* was in advance of the move to the network in my mind a *main point in favor of joining the network*. 

  


  [1]: http://academia.stackexchange.com/
  [2]: https://tex.stackexchange.com/
  [3]: http://mathematica.stackexchange.com/
  [4]: https://stats.stackexchange.com/