According to Fangraphs, Tim Hudson has had the most productive fastball in Major League Baseball this season. The statistic wFB refers to how many runs above average a fastball is -- the prefix "w" in front of any pitch type (SL - slider, CU - curveball, etc) denotes this. Hudson's fastball currently has a 36.4 wFB compared to the second place Ubaldo Jimenez's 27.8 mark.
Hudson's fastball is really a sinker and is not thrown with nearly as much speed as Jimenez's or most other of the top ranking pitchers. What Hudson's fastball does is move down and often out of the strike zone, forcing batters to make weak contact on grounders. This is evident by the fact that Hudson also leads the majors in ground ball percentage at 65.2%.
The statistic used by fangraphs has only existed since 2002, and only three pitchers have ever posted marks higher than Hudson's current mark of 36.4. Those players are Jason Schmidt with 42.4 in 2003, Dontrelle Willis with 44.1 in 2005, and Derek Lowe with 46.8 in 2002.
If the rotation is not altered the rest of the season, Hudson will start nine more games. If Hudson continues on his current pace, he will end with a 49.5 wFB, which would be tops amongst any pitcher since the statistic has existed. Staying on this current pace would be very difficult, but over 25 starts he has managed to dominate with the pitch more than any other pitcher -- I don't see any reason to expect his sinker to decline.
Basically, Tim Hudson is on pace to have the most effective fastball since, at least, 2002. This is a pretty history year Hudson is having with this pitch. He has always been known as a sinker-baller and ground ball pitcher, but he has never had this much success with it and neither has any other pitcher since the start of George W. Bush's first term in office.