Joomla is, in my opinion, the top CMS. It is also my opinion that Simple Machines Forum (SMF) is the top forum software. These two packages can work together, however it is quite a task and depends on how you want to build your website.
The SMF website has available for download their own version (I believe it is made by Oristo) of a bridge to merge SMF and Joomla. This bridge does work and works with advanced mods for SMF, such as the Arcade mod. The part that is lacking, however, is the integration between Joomla’s user database and SMF’s user database.
JoomlaHacks.com, on the other hand, has a large SMF bridge project available for download. This package re-writes SMF url’s and needs to patch some of SMF’s core files from within Joomla. If you have Community Builder (CB) installed with your Joomla, this bridge can make SMF link to your CB profile and messaging systems.
There are a few things lacking with this bridge, however, and that is that SMF notification messages and Joomla / CB notification messages use two different messaging systems. What you have to do is decide if you want SMF or CB to control the private messages. Remember to disable private messaging in the other system, as you users will be unable to get to it. You also have to decide if you are going to use the forum’s user profiles or Joomla / CB ’s user profiles.
For most projects I would recommend the JoomlaHacks.com version of the bridge instead of that on the SMF website, umless you need to use advanced mods with your SMF installation because the JoomlaHacks.com version can cause some bugs with the mods.
Also make sure you plan out what you need and how it should work and which system should be displaying what.