BJJ all the way. Its awesome, the best when it comes to ground grappling. And all the BJJ guys I've met have been great people too.
If there is no BJJ dojo around, try one of the following:
Sambo - sambo is like freestyle wrestling with some submissions allowed. A lot of emphasis on takedowns and leglocks.
Judo - probably the best style to take a gi-wearing opponent to the ground. A lot of emphasis on throws. Not as effective with no gi. It is also an olympic sport, so it has a wide following. But pick the dojo carefully, a lot of bad ones around, especially outside of Japan.
Wrestling (Freestyle/Greco)- Also an olympic sport. If you want grappling with no gi, this is a good pick. It is more of a power game than technique, so a good muscle mass is recommended
A lot of emphasis on takedowns and position control on the ground.
The most competent fighters I've met started off as freestyle wrestlers, moved on to BJJ and cross trained in a striking art like boxing or Muay Thai. Master the wrestling-BJJ-kickboxing combo and you'll be a living demolition machine