In 21.5 you can kinda run any race grade esc. Like the HW 80a pro stock spec is popular because it has all the features but like a quickrun 80a being much cheaper wont have nearly the same performance.

Hoosier wont steer you wrong, I live in ohio but I'm down at Hoosier often and Angie knows what the racers use and what they like. Basically anything they sell will be just fine. I actually have a 80a pro stock esc as a spare and am using it right now in my 13.5 4wd, and its not holding me back but my motor sure is. So 21.5 will be fine, and many guys run it even in 17.5 for less weight. But usually I run reedy stuff as anything they make is solid and easy to tune. Fantoms is great as well and I'll be trying out one here soon.