Are you asking for a subject or a thesis for a subject that you already have? Well, a good thesis statement should include a little insight on your opinion as well as setting the tone/outline of your paper.
Do you need some good ideas for argumentive essays? If so, I would start with something thrat your interested in that has at least two sides. Are you a sports fan? You can always argue about how your fave team should/should not have won the championship. Include statistics, reasoning and other professional opinions.