Definitely Big Brother US
I was slightly into RTV before it, like I watched a bit of Jersey Shore when it was around, I had watched some of Tila Tequila's A Shot At Love after hearing about it from like middle school girls in my grade
and would occasionally watch some other RTV like Real World and whatever but I never got into any of it that much until BBUS...
Now I watch BBCAN, CBBUK (and civilian a bit), Survivor, just started The Challenge, and I'd give most other RTV shows a chance