Out of all the physical specimens the league has, Barkley is probably the most unusual. A guy 6'4" who can dominate the PF position for more than a decade? He can handle the ball like a guard, he can run the floor, he can rebound, and is basically a terror out there.
O'Neal is big and agile, but there was a guy called Wilt who was as big and more agile, James is a modern day Robertson, Jordan has Kobe and Carter as his followers, Garnett has the Chandlers and the Aldridges as followers. We are not talking about skills and accomplishments, but as a physical specimen, Barkley is as unique as any, we have never seen anyone similar ... unless you want to count Clarence Weatherspoon ....