On my Mr. Minute-Mech Sentry Bot (a Minute Man themed big guy stationed at The Castle's front entrance) I used the cluster bomb launchers. His aim is questionable, but the damage on a hit is impressive.
My other Sentry torso bot, Grim Chaos, is mounted on Assaultron legs with armor that makes them look like hooves and has the Ahab head armor and twin stealth blades. I put the frenzy gas launchers on him and while, once again, his aim is kinda derpy, when he hits the effects are great. The melee weapons keep him from stealing so many of my kills and when the gas hits on enemies outside of his immediate reach, we get an instant distraction while we move in for the kill.
The third option, shoulder mounted Fat Man launchers, are a HORRIBLE idea. I don't even need to test them out to know I'll probably die instantly the moment my robot even gets a wiff of so much as a Radroach.