A trained German Shepard with it's unmatched protective instinct can take down pretty much anything in it's size range.
I had a German Shepard all through high school. The baddest motherfricker around. He was a straight killer. My neighbor's cat slapped the shite out of him once and he took a step back, then took a step forward and bit the cat damn near in half. Then calmly just trotted off.
I've seen bobcats and they are one the coolest wild animals around. That said, this dog would've fricking destroyed a bobcat. The GS in the above story was fricking soft. Mine played SEC ball.