If this is Conquest, then Rep is usually the way to go for all, but war. In Always War I go with Monarchy. I never use Democracy as you have to make an additional switch and also research some optional techs.
Commie is a decent form to use if the empire is large, but again you need to research a tech or two that you do not need and switch again.
Also it can get large enough that the corruption starts to hurt the core.
You end up making extra structures (Police Station) that cost time and shields and support.
In most games I do not research either Demo or Commie and Facism is just awful.
Demo also is very problematic in war times, I cannot remember the last time I used it.