Tharsis Tholus. Is one of the steepest shield volcanos with average flank slope of 10 degrees (followed by Ceraunius Tholus which is 10 percent less steep (average flank slope is 9 degrees) than the Tharsis Tholus) and has the steepest slope of angle 16 degrees near its summit. Ascraeus Mons and Elysium mons also have steep slopes compared to Tholus, But when it comes to average flank slope, Both are 33% less steep (average flank slope in 7 degress) than Tharsis Tholus.
Talking about the longest line of sight, as you mentioned in the question, due to the size and shallow slopes of Olympus Mons, an observer standing on the Martian surface would be unable to view the entire profile of the volcano, even from a great distance. The curvature of the planet and the volcano itself would obscure such a synoptic view, similarly, an observer near the summit would be unaware of standing on a very high mountain, as the slope of the volcano would extend far beyond the horizon, a mere 3 kilometers away. Neither Ascraeus mons nor Elysium mons (Even Tharsis Tholus) will allow you to see an actual "horizon" because of the "low" steepness and the curvature of Mars. Yes, The longest line of site can be seen in Valles Marineris (Melas Chasma?). With help of some astrogeology and math, taking the reference site as Melas Chasma (the widest segment of Valles Marineris, you can actually see as far as 255-279 (+/- 8 percent) Kilometers.