For the last 6 months, Think Secret has had the best track record with it's rumor reporting. Plus, they report on a wide variety of rumor topics - hardware and software.
Mac OS Rumors (mosr.com) used to be the best, but lately, they've started to speculate about projects so far down the pipe that it all sounds like they are blowing smoke up your arse. I mean, even Apple doesn't really know what's going to be released 1 year from now. They have projects in the works, but when they are in the very early stages, they can go many different ways.
MacRumors does little more than parrott what is already out on other websites.
RailHeadDesign, while not specifically a rumors site, does have inside info from time to time. Most of Maury's hardware rumors usually don't come to fruition, but his software rumors are usually dead on. How Maury gets around his NDA is beyond me.
SpyMac has got to be the worst. That whole iWalk debacle left them with zero credibility in my book.
On a nostalgic note, I really miss AppleInsider. Up until about 18 months ago they were the best rumor site. They broke the Cube story days before it's release, and were dead on about 75% of the time. It's a shame to see the site nothing more than a forum now.