I used Cloudmark Spamnet last year when it was still free. Easily the best, but now carries a subscription fee.
Next, I tried POPfile. That's not so user-friendly. There is a Outlook front end called "Outclass" (or something) that makes it easier to integrate with Outlook, but still.
I now use SpamPal. Filters mail by origin, based on scripts, established public blacklists and your own whitelists. Probably the best free spam filter available.
But if I were to fork out a couple of dollars a month, I'd use Cloudmark.
Craig