Sopranos. Groundbreaking, maybe, bot so boring.
I think Sopranos gets a bit too much credit for being the groundbreaking series. First off, the first HBO drama is Oz, which is every bit as challenging and groundbreaking as Sopranos (well, let's pretend the 5th season didn't happen). Sopranos had the good fortune to be groundbreaking AND popular. It caught the lightning in the bottle.
But as far as the proto-show which "changed everything about TV", that show is clearly Twin Peaks. Without Twin Peaks, the Golden Era of TV doesn't happen. At least not the same way. I like Sopranos, but I think it should be judged on its merits rather than getting bonus points for creating the Modern Era. Unless we're going to give the same credit to Homicide, Twin Peaks, Oz, and let's be honest, the Simpsons.