rating in portal announcement

[ZiNgA BuRgA]

Unless you really need the ability to have it in multiple forums, I'd recommend sticky threads personally.  Announcements and threads are handled differently in MyBB, so it would take some work to get ratings on announcements.
Ultimately, it might even be easier to have a thread appear in more than one forum than a rating system for announcements, although the former has a number of issues (the thread will still only belong to one forum, though shown in multiple).