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Until recently I would have recommended IPB. You do have to replace some of the XOOPS 'core' PHP files to install it, i.e. it does not have a point-n-click installer like other modules. I had it up and running in an hour (with backups of the files I changed) but you will need to upgrade the core files again if you upgrade Xoops. You also have to create a separate IPB theme if you want it to match the XOOPS theme.
On the plus side it is very good and it's the only 3rd party forum module that had solid support here on xoops.org. AFAIK there are two IPB modules. The 'free' one is available from the module repository. An IPB 2 module (which requires a licence fee) is available from bbpixel.com.
I am a big fan of PHPbb and there are a couple of PHPbb modules for Xoops, but sadly there is little or no support for them. As jdseymour says, forum modules are prime targets for hackers so you need a module with an active development team behind it.
NewBB 2.0 (worth a miss IMHO) is the forerunner of CBB. CBB has pro-user features and good support too. It would be my choice at the moment.
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