Since I brought this up in April, I'll do it again here...
Ubaldo Jimenez- 5-1, CG SHO, 39/12 K/BB, 0.64 ERA
Roy Halladay- 3-3, 2 CG SHO (one Perfect Game), 37/9 K/BB, 2.15 ERA
Yes, I left Dallas Braden off this list. His Perfecto was his only win this month, and outside of that one game he's 0-3 with a 3.19 ERA. Tim Lincecum currently leads the league in K's, but his line was 1-2, 30/23 K/BB, 4.95 ERA, which is hardly worth the consideration.
So, Halladay or Jimenez (or do you have another option)? Personally, I'm taking Jimenez again. The Perfect game was a big deal, but I don't think you can put that much stock in a single game that you are willing to overlook the overall consistency of another player.