There is a new one that has sprung up which looks pretty good, though that's all I can say about it. It looks like it's currently in a development stage, so it might not be best for a serious forum. However, it does look pretty easy to skin, and I like the way the posts and things are laid out. Check out MyTopix here: http://www.jaia-interactive.com/support/
No doubt then vBulletin would be the best choice. You can purchase the license and there will be no annual fees.
Vanilla is perfectly clean and valid code...it is not hard to mod at all... it just depends on what you are used to