I′ve encountered them all and the most fun was the locked up one in Evergreen Mills. I was at a safe location high up and I blew the gate open and let him out amongst the raiders!
The most scary encounter I had was at the GNR plaza. He came as a total surprise to me! Luckily I had a Fatman with me and scored a direkt hit with it, and then the big green mean meat machine was easy to take down

I just wondered, did you kill off all of them?