I voted Geipel, but I may have made a mistake.
Over the past two seasons:
Greipel has
42 wins, 5 are GT stage wins, 10 are PT wins and he won the points classification at the Vuelta.
In comparison Farrar has
17 wins, 4 are GT stage wins, 5 are PT wins.
So on the surface Greipel looks better. However, head to head over the past 2 years it looks like they've raced together 28 times or so. If you look at those races, and the times the two came to the finish for the win, Farrar has beat Greipel 10 times. Greipel has beat Farrar only 6 times.
And when Farrar beats Greipel he brings it home for the win more often, 6 of the 10 times Farrar has beaten Greipel he's won the race outright. Of the 6 times Greipel has beat Farrar he's only won the race outright 2 times.
Petacchi is pretty impressive too. His best years were 2002-2007. But in the past two years he has
20 wins, 6 GT stage wins, 1 PT win and the points classification at the Tour de France.
I'm kinda stumped