There are a number of random encounter hot spots throughout the game, but I've found a new one that has been even more of a hot spot than any of the others in my experience. Out of 15 times that I've gone there, I've had 15 random encounters. Most of which I've never even had in the game before. Fast travel to Drumlin Diner, turn around, and head basically due south along the road. You'll quickly come to a small house. The encounters will happen inside of it.
So far I've encountered synths attacking raiders there, some dead raiders with Pickman's calling card, and just a little bit ago I found two people named Fred and Angie holding a third guy named Jules at gunpoint. Jules had been revealed as a synth and you have to choose whether to let him go, or execute him. I let him go.
I've also successfully recruited both Doc Anderson and Ron Staples here, and had a number of other interesting encounters. I highly recommend checking this place out. I regularly stop by Drumlin Diner to offload some water anyway, so I've been making a habit of checking this spot out every time.