No. 2: Roger Clemens
Roger Clemens, like Pettitte, spent three years in Houston. He came out of retirement to sign a one-year deal in 2004 before re-upping again in 2005. He then (what?! emerged from retirement once more, this time mid-season, in 2006. And you won't believe what he did the next summer at Yankee Stadium, either.
Clemens was otherworldly in his Houston tenure, going 38-18, recording a 2.40 ERA, winning one Cy Young, and somehow finishing third in 2005 (1.87 ERA, 226 ERA+, 2.87 FIP).
He made seven playoff starts across 2004 and 2005, and while he wasn't as dominant as he was in the regular season, he remained a huge piece of the Astros' first World Series trip in franchise history.
He already had a Hall of Fame resumé before signing in Houston, and landing a player of his caliber was the biggest splash the Astros had made in decades. 25 years, to be exact. Only one signing in franchise history was bigger than The Rocket.