#2 David Bednar
David Bednar would be quite a splash for the Astros. Over the last three years, he's quite possibly been a top-three reliever in baseball (2.25 ERA, 1.06 WHIP, 11.3 SO9). He's not a household name since he plays for the Pirates, but even on a losing team, still managed to lead the MLB with 39 saves this season.
With Hector Neris, Phil Maton and Ryne Stanek all hitting free agency, the Astros have to address their bullpen. A back of the bullpen consisting of Bednar, Bryan Abreu and Ryan Pressly is simply obscene.
Bednar would come at a cost. If Pittsburgh decided to move Bednar, they'd be expecting a lot back. A couple of controllable starters like José Urquidy and Brandon Bielak, a young and talented outfielder like Jake Meyers, and multiple prospects would be heading back to Pittsburgh.
But if Houston wants to win again, a lights out reliever like Bednar goes a long way to improving their chances.