There are many superheroes. But Sheldon from The Big Bang Theory loves The Flash very much. He loves Batman too. But Why? What's the reason?
From collider.com on Comic Con 2012:-
A child asks Parsons, “Why does Sheldon like The Flash?”, Parsons threw the question to the showrunners, and Prady responded that Sheldon liked the Flash because Sheldon was bullied a lot and the Flash was able to run away really fast.
In-universe, The Flash is very pro-science, specially physics and he is (they are) nerds turned super-hero who can travel faster than light. He is the fastest superhero and one of the oldest too.