Well, I cant say that I have a favorite supplement, but I will touch base on each of them on what I like the most and least. (my opinions maybe off because I have not got my friends to play it yet.
Warmachines of 2089
Likes: The warmek workshops adds a lot of info and is well laid out. And of course, the multitude of warmeks is awesome.
Dislikes: Would have liked to see the warmek design portion a little more streamlined (in the core rule book as well). But this is something that I have not used much yet and it might be a good layout.
Likes: Great description of every country and giving a great platform to launch a game from and fully .
Dislikes: Because of the novels and other supplements, would like to where these countries where planning on doing. I.e. what is Canada's next planed action, try to turn true neutral, align with the EF, or align back with the USA? Of course all plans would be subject to change based on others actions, but an attitude toward their next action would have been nice as well.
Likes: Great description on new classes, how to use non-warmeks in full out battle, new equipment and technology. This is probably my favorite if I had to chose just one. This is just a well done supplement!!
Dislikes: I would like to have seen more example vehicles.
Likes: Did a great job on giving a new role for airplanes, space. Setup and descriptions for using space was superb, as was the classes and new space agencies.
Dislikes: This supplement kind of went off all on its own. It almost seems like you either have a warmek campaign, or a space campaign. So I would have liked to see more meshing of the two, like more space capable warmeks or something.
Overall opinion: Awesome Job!! Cant wait to get any more supplements, keep up the good work!!