"I reckon so" is what I use more than any other. I also use "it's amazing they lived" which refers to the object having been beat nearly to death with a stupid and/or ugly stick.

My Dad's was "you may be big, and you may be dumb, and you may be ugly...but other than that you don't have much going for you" and it was usually directed at me.

My daughter uses "drive it like you stole it" and I'll probably incorporate it into my daily use.

What would Porter Rockwell do?