Depends. The catch-all spot is
Talk:Main Page. The spot for suggesting policy and other far-reaching changes is
HRWiki:Da Basement. Then there are specialized locations for discussion:
HRWiki talk:Standards,
HRWiki talk:Projects, and
Talk:Strong Bad Email, just to name a few. Try to find the most specific talk page you can (in other words, if you're trying to improve the article on one specific toon, you should use that toon's talk page; if you're trying to improve a whole project, find the talk page for the specific project).