Anne of Cleves did absolutely nothing (including: not even had sex with Henry) and was granted a sh**tload of real estate. That's some genius.
Also, the less-than-ideal way in which her marriage turned was the final spark for the downfall of Thomas Cromwell. HAHAHAHA HenryWallaceVP pwned.
Maybe I should've voted for Anne Boleyn, as without her England would have remained clouded in popery and superstition. Plus, she didn't cost Thomas his head but rather the other way around. She was also a fascinating, highly intelligent and cunning figure in her own right, and as The_Doctor points out she gave us Good Queen Bess.
I'm very aware of what happened to Cromwell as a result of the Cleves marriage, yes. The third book in Hilary Mantel's Wolf Hall trilogy, last year's
The Mirror and the Light, is all about Thomas's tragic downfall. A terrific book and series by the way, which I'd highly recommend reading. I think you might actually like Thomas if you got to know him a little better. He was extremely fond of Italy and Italian culture, and the years he spent there as a young man deeply impacted him for the rest of his life.