It all depends on the type of community as to what mods fit best, but here's a few all-rounders that I would suggest using on any vB forum...
NoSpam! - NoSpam! allows you to specify a set of questions which members are required to answer correctly at registration, eliminating the ability of spam bots to register at your forums and post unwanted messages.
and last, but most importantly vBSEO, although this is a separate product with its own price tag at $150 it helps your forum SEO massively with an array of useful SEO features.
If you can't afford vBSEO then it's worth at least getting vBSEO :: Sitemap Generator which is free as an addition to the main vBSEO product, this will generate a Google & Yahoo Sitemap for your Forums.