Out of the box? I believe it should be SMF with SE Friendly URLs. But PunBB looks good as well thought I have never tried it in production.
Hands down Drupal. The integrated forum is visually ugly and needs a lot of theming work, but Drupal has the best clean url structure built in. My forum urls all look like this: mysite.com/forum/subforum/subject-title.html subject title is whatever the thread is called by the creator. My forum brings me more traffic from Google than the rest of the site. Sometimes I rank for things that have just been posted, due to the keyword rich url
No. For me it was the reverse issue. I have built all my sites with Drupal and they didn't have forum plugins for a long time so I was forced to use the drupal forums (which lack a lot of features that you need to install modules for). The reason I stuck with it is its perfect integration into the cms (I can have the sidebars enabled and it blends perfectly with the theme, so I only need to theme one software package). It makes a lot of sense if you start from scratch to go with drupal, but it cannot be used as a forum module and quite honestly is inferior in many ways, except SEO where it outperforms anything else (maybe vBSEO is similar but it costs money). If you like, you can PM me and I'll send you the url to check out (in case you are wondering about Drupal).
MiniBB - www.miniBB.net I use it on a few sites that have smallish forums - such as support for the site etc. I use it not only for the SEO advantages, but because it is the easiest forum to skin and integrate into an existing site. It takes a little bit of time to understand how to set it up but it is well worth it. First ime, It took me about 4 hours to install, configure, add some plugins, do a few minor hacks and skin. The 2nd time, It took me about an hour to do the same.
SMF sucks in terms of SEO. The topics don't get indexed well Vbulletin.com 211,807 posts http://www.google.com/search?hl=en&q=site:vbulletin.com/forum&btnG=Search 368,000 pages indexed. simplemachines.org 118,547 topics http://www.google.com/search?hl=en&q=site:simplemachines.org/community&btnG=Search 22,500 pages indexed. Remember now, There may be a ton of posts, but each topic counts as 1 page so while vbulletin.com has more "posts" it has only about double the number of pages. So it should have only had 45,000 pages indexed, but nope, it has about 350K
thanks for the useful info, this might add juice: phpbb has 344,000 listed at google http://www.google.com/search?hl=en&q=site:phpbb.com/community&btnG=Search so does that mean phpbb is doing well incase of SEO? how do u count the posts anyway?