No. 6: Charlie Morton
When the Astros signed Charlie Morton to a two-year, $14 million deal with up to $5 million in incentives heading into 2017, who could have foreseen what a massive move that would be? Morton came to Houston with a career ERA of 4.54 in 893 innings of work.
With the Astros, Morton turned into a terror. Morton was 29-10 (including a league best 15-3 in 2018) during his two seasons in Houston, posting a 3.36 ERA and 10.4 SO/9. He evolved from a groundball pitcher into a strikeout machine.
He, of course, was great in the 2017 World Series, when he famously closed out Game 7 with four relief innings of one-run ball to get the win. While injuries kept the Astros from repeating in 2018, his dominance in the regular season is hard to forget.
Morton had really been nothing more than an innings eater for Pittsburgh before signing in Houston, but quickly grew into a dominant starter during his time here. It's entirely possible Houston doesn't have a ring in 2017 without Morton. It wasn't a lucrative deal, but Morton is undoubtedly one of the most important free agent signings the Astros have ever made.