I don't think any of the previous forum plug ins are that great compared to something like a V-Bulletin, but I did see this one and I'm am actually going to try it out today: http://simplepressforum.com/
A wordpress plugin that allows forum capabilities. Almost like interfacing something like phpbb or vB as a page on wordpress. I want to add a blog w/sidebar widgets on my forum's landing page.
I don't know of a forum plug in that does that, however, V-Bulletin 4.0 Publishing Suite has blog capabilities, and sidebar blocks on the home page, as well as, a sidebar on the forum pages.
I have that plug in installed by the way, if you want to take a look at it. I'm just using it for support questions...nothing to rubust, but it seems to be pretty much what I needed: http://clusterfox.org/support/
A best forum plugin or theme doesn't exist. It all depends on what you need to do with that forum. You can use vBulletin, phpBB, Vanilla, SMF, Invision Forum as options for creating a forum with scripts that are built especially for this purpose. I recently created a wordpress theme that will act as a forum here http://thegforums.com/ But like I said, it all depends on your needs. So the best forum option is the one that satisfies most of your needs, the rest can be done with a bit of custom editing.
You could use rss enhancements plugins for both phpbb(free) and vbulletin, and use rss widgets to list top posts, most recents posts, top users etc. And for wordpress back to phpBB I have some plugins I am developing and have developed such as rss2phpBB http://www.rss2phpbb.com, and am conceptually developing large scale products for both phpbb and vbulletin due to release this year. WPGLamour: The Gforms looks pretty cool!. Gonna bookmark that one.