Our spacecraft have rarely visited Mercury, for reasons obvious to those who know orbital mechanics. The Mariner 10 visit to Mercury was revolutionary by using Venus for the novel slingshot maneuver, placing it in an eccentric solar orbit to enable the probe to "visit" Mercury every other orbit. It didn't have enough fuel to orbit the planet.
The MESSENGER craft used two Venus and one Earth slingshots to get to Mercury. That was surely a lot of work. It took a few "visits" to Mercury before orbital insertion around Mercury in 2011, about 7 years after launch.
I thought perhaps that Mercury's high (solar orbital) speed would enable a slingshot to be rather effective for other purposes. Instead of a craft trying to "visit," suppose it was used for blasting away to the outer planets or beyond. Would this be effective?
My question is this. If you send a probe on a slingshot around Jupiter, for example, back to Mars or Venus or Earth, for example, could you point it to Mercury as a better accelerating slingshot to the outer solar system? Or is coming back sun-ward to the small, inner planets just a waste of time and resources? Calling this a "momentum transfer" seems to indicate that Jupiter, Saturn, and the outer planets are the best, but other factors are at play like atmospheres: Mercury was approached to "16 miles above the surface" in 2015, whereas that's impossible for the outer planets. And I see an advantage to being near the sun with a high inbound velocity.
So I am leaning toward Venus being a better slingshot candidate than Mercury based solely on its mass.