The Texas Rangers and the Houston Astros have the fourth best rivalry in MLB right now, according to Anthony Castrovince with MLB.com, who has a rundown of the top 10 rivalries in MLB. Castrovince puts just the Cubs/Dodgers, the Cubs/Cardinals, and the Yankees/Red Sox ahead of the Silver Boot contenders.
I’ve generally been dismissive of the notion of a Rangers/Astros rivalry, but with several bench-clearing incidents in recent years, and the Rangers and Astros being involved in division races the past few years (well, 2015 and 2016, anyway -- the Astros ran away with the West in 2017), as well as the Astros winning the World Series last year, there does seem to be a greater sense of a rivalry developing between the two teams.