Hotter N Hell is the big one, 14,000 or so cyclists. But it can be hot (duh) and the route isn't particularly scenic.
Houston's biggest rides are the BlueBonnet Express and the Katy Flatland
Austin has the Austin Armadillo
I also enjoy the Goatneck which is in Cleburne.
Those are some of the bigger ones I am aware of. There is a Tour de Houston this year (they skipped last year) and a ton of smaller rides.