probably none of them, in my opinion -- unless you want to screw over your link partners heh. you need to nofollow any of the internal links except maybe the login screen.
Put nofollow tags on all the pages that don't bring you search results... like: login register members faq tell a friend contact us etc. That will put more link power to the pages that do bring you visitors. And, of course don't put nofollow on any link exchanges unless agreed upon by both parties. It's sneaky and slimy. Simply bad manners.