All fans of the maximus that is gluteus, listen up -- one of the best butts in the world is about to pick the best butt in sports.

We got Jen Selter, the Instagram model that has turned her butt into big business (11 million Instagram followers) and asked her to weigh in on pro athlete ass.

Her choice was one of the usual suspects ... and definitely a Hall of Fame rear ... but things got really interesting when we asked who had the better rumpasauras outta Tom Brady & LeBron James.

We're sure you guys care about all that ... but in case you don't, there's a gallery of Jen below. You're welcome.