Although Olympus Mons is the tallest mountain in the solar system, its slope is so gradual that the peak cannot be seen from the plain; it is hidden by the planet's curvature (flank slope = 5.2°). The answer to this question suggest Tharsis Tholus with average flank slope of 10° followed by Ceraunius Tholus with average flank slope 9°. Tharsis Tholus is considered one of the steepest volcanoes, so are there any mountain/volcanoes steeper than Tharsis Tholus?
I searched for a bit and found Boösaule Montes which is the tallest mountain in Io. The mountain has an irregular morphology, with a relatively gentle slope throughout much of the mountain except for an abrupt scarp on the peak's southeastern margin. According to Schenk et al. 2001, this scarp has a height of approximately 15 km and a slope of 40°. But, I am not sure if this is to be considered as "average flank slope" as it is only steeper on one side.