Though I have not followed the Astros as long as you have, I would have to agree PopTop, though it is pretty close
I feel Bagwell both through pure talent and performance with the team is the best Astro ever. I understand the arguements for Nolan, but Bags has a larger resume in Houston, due to playing a majority of the pro ball with the club (though I often neglect the Red Sox minor league years when looking at his professional career).
As far as greatest Bagwell and Biggio both get large amounts of credit for playing there whole major league career with Houston, and both being great team leaders in that time. One of the places I think Biggio sets himself apart was when he resigned in the late 90s (maybe 1996). If I remember correctly after a very good year, he took a large pay cut in order to stay with the Astros, even though another team (Indians possibly) was offering him a more lucrative deal. He wanted to be a leader of the team, and he believed in Houston. Also Biggio's philanthropic work in the community helps make him the greatest Astro in my opinion.
Lets Go Micah!