Hi there,
Honestly, the ones to focus on include the title tag, the meta description, and the robots. If you have a non-English language site, then consider the meta content language and meta language tags. The keyword tag does nothing for rankings. In fact, the meta description impacts rankings very little as well, but assists in providing a marketing message in your search result that can impact a click through to your site. The title tag is probably the most significant on-page ranking factor, so pay special attention to how those are executed sitewide.
Here is a good review in Webmaster Central : http://www.google.com/support/webmasters/bin/answer.py?answer=79812
Hope this helps!