"Most difficult", as in the minimum $\Delta v$ required, the objective being capture into an orbit around the object.
Looking through some Delta-v maps, I would suspect the answer to be some inner moon of Jupiter or Saturn, but those maps are not including every solar system object, they don't always take full advantage of all the routes available in the patched conics approximation, mostly ignore flyby routes, and often simplify the aerobrake options available.
Thus, I'm looking for the worst case when also considering the more complex trajectories. It should be practical to execute within a couple of decades though.