I think that the best sport in that definition is MMA, for the following reasons:

* Sports are, historically, training for war during peace-time. Beyond that, they are about competition: two people or teams physically competing for dominance. MMA gets right to the heart of that.

* There's a significant amount of strategy that goes into MMA and combat sports in general, and you pick it up fairly quickly.

* It had one of the most rapidly-expanding viewer-bases of the last decade or two.

* Like others have said about soccer, it has a low barrier to entry, except with MMA it's even lower: you just need to have 4 functioning limbs (which is admittedly still a barrier, but that's a barrier to soccer and hockey as well).

I think that the 2nd to last point really hits home, as even people who *were* familiar with other sports started flocking to MMA in really high numbers.