Linux.
Or, just don't install Spyware in the first place. Once you get it, it's very diifficult to get rid of. No spyware program will get 100% of it.
Ad-aware is considered the best, according to security professionals
S&D is a close second
Microsoft's is a distant third. I don't recommend it, though, because they only detect spywares whose companies aren't paying them (Gator/Claria, one of the worst spywares out there, isn't detected by Microsoft. And there are others, too.)
Of course, those are pretty arbitrary rankings. I'd suggest having both S&D and Adaware, since neither will find everything.