One result of the Astros' upcoming switch to the American League is that the club will have a new position to fill the next time the marketing department decides to drum up excitement by announcing an ALL-TIME TEAM. It is quite possible that by 2023, Jon Singleton, Telvin Nash, Chris Carter, J.D. Martinez or some other hitting specialist will have put up enough numbers to be considered as the franchise's top all-time Designated Hitter.
However, in the event that no DH rises to prominence for the American League Astros in the interim, the leader in the clubhouse at the moment is... Carlos Lee.
Lee played 27 games as the Astros' DH during interleague play. He leads Houston's DHs in hits(25), home runs(5) and RBI(18). Lee's batting average as a DH for Houston was .243.
A few notes about Houston's DH history heading into their AL experience:
In 120 games at American League stadiums, Houston's DHs hit .270 with 17 home runs and 58 RBI.
Jeff Bagwell failed to hit a home run in 10 games as a designated hitter.
Bagwell was 1 for 6 as the DH in the 2005 World Series.
You can check out the unofficial Astros' historical DH statistics page here.
#-On one occasion, Carl Everett pinch hit for DH Jeff Bagwell. I didn't include Everett's stats. on the linked page in my research. He was 2 for 2 as a substitute DH.