I'd have to say Tim McKeon then XD Since he wrote most of the episodes that I really, really enjoyed, namely:
Room With A Feud, Squeeze The Day, Neighbor Pains,
Infernal Slumber and I Only Have Surprise For You; the episodes in bold being my top favorites because they're sheer insanity. I mean, using all the main characters in a way that isn't cheap-skate combined with a hilarious story that ends off nicely? Hands-Down win. XD
But I love all the writers of Foster's, I really do. Even those for the supposedly "bad" episodes; they all contributed to what Foster's is today, after all. (I personally don't see anything wrong with the Suffering Character Episodes as long as the ending is satisfying. XD "When There's a Wilt There's a Way" remains my favorite Wilt Torture episode because of the sheer amount of fail that Wilt goes through. XDDD "NNNNNNNNN!!")
And the pilot for Foster's still remains my favorite in shot-direction and plot. The choice and style of shots was one of the things that struck me immediately when I first watched it.
All the writers are insanely awesome either way. Writing is hella hard. That they can write for Five freaking seasons is quite something, isn't it?
Yay for Foster's writers!