[First off, a disclaimer: I work at a competitor to Microsoft in the anti-malware space, so please keep that bias in mind when reading my reply. AG]
As far as core features like prevention and removal goes, Microsoft's anti-malware line (Microsoft Security Essentials/Windows Defender, Forefront/Intune) are fairly decent choices.
Where the free offerings tend to fall down are on the features like configurability (high-granularity of features), remote management, centralized logging and so forth. There's also a matter of things like design philosophies (what techniques and technologies the company uses in architecting its software), employee allocation (which affects the product in various ways), threat prioritization and the like which make subtle (but often noticeable) difference in how an anti-malware program operates.
I have to think that they left out some bits from MSE in defender for windows 8 so the rest of the antivirus companies didn't cry antitrust.