Why does this website deserve to win?:
Proformative relaunched the website in mid Feb 2012 with new theme, new features, and a fresh focus. Very integrated search, follow users/threads/topics, reviews, very simple, fresh looking. The site is a "professional question and community" site oriented towards Corporate Finance, Accounting, and Treasury professionals.
Get past the home page (which encourages registration) by just clicking on "about" at the bottom then you can enter the entire site as an anonymous user.
The business directory is very nice where you can see an intelligently laid out mini-site for each business. Example: www.proformative.com/b2b-directory/host-analytics
How It Was Done (optional):
SOLR search; users can follow things - which controls their home page stream and digests; activity log for users that feeds home page streams; customized basic theme; many (>80) Drupal modules; dozen custom modules; very nice use of organic groups for business mini-sites; members can rate/review businesses; Uses Pressflow, memcached, APC.
Heavily integrated SOLR with almost everything including distance searches, custom sorts, dual searches, etc.