Here's actually what I think are the 14 most important, either because design care is needed, or because they're common, or because they are early-on in user conversion...

notifications
(in general)
new reply to your support ticket
someone added you as a friend
new comment posted
your content was liked
new topic posted
new private topic opened with you
topic replied to
it's a friends birthday
notification digests
newsletter with automatic what's new content
recommend website
lost password reset confirmation
join confirmation