I wouldn't say basketball is the best sport for pushing people to their physical limits, but it is definitely the best sport for showcasing the strengths and physical traits of individuals.
Anyone can play basketball in a way that takes advantage of his/her unique skills and characteristics. Height (or lack of height), weight, athleticism, intelligence, leadership ability, power, instincts, coordination and many other characteristics all factor into a players ability to play the game successfully.
NFLers are probably better athletes on average but I think athleticism may not weigh as heavily on the success of NBA players.
Size definitely factors into success in both sports. You can pit a team of 250lb people against a team of equally athletic 150lb people and its easy to guess which team most people will bet on.