Well, of course it has to be Karl Rove, or Turd Blossom, as our Commander in Chief calls him. While I am happy to see him go, I must honor his previous achievements -- he's committed enough bastardly acts over the past 15 years to fill several books (see below). He's second to Dick Cheney in responsibility for the mess known as the Bush administration.
Rove's worst contribution to the world is his conflation of politics and policy. Every policy enacted by the administration is orchestrated for maximum political benefit. Rove doesn't let Bush wipe himself in the bathroom without weighing the political ramifications of both wiping and not wiping. I guess I might admire that approach as a cynic, but it doesn't seem like the wisest or most efficient way to govern a nation.
Although Rove has been hailed as "the architect" and "Bush's brain," his electoral successes haven't been as great as the hype. Let's face it, the Supreme Court decided the 2000 election, and riding the coattails of 9/11 won the 2004 election.
Finally, as Kathleen Reardon has already written, haven't we heard this "spending more time with my family" excuse for resignation just a few times too many? Rove's son is in college. If he really wanted to spend more time with his family, why didn't he resign before his son's summer vacation? Unfortunately, I'm sure we haven't seen the last of this bastard.