Been playing Dark Souls and Armored Core V. Satisfying in Dark Souls is basically any riposte or backstab, but particularly the Uchigatana and Bandit's Knife. Uchigatana is also very satisfying just with R2 attacks and upward slices, and causing bleed on the drakes in Valley of the Drakes is just awesome. Also headshots with arrows that cause the foe to go flying, Emit Force on enemies that will die from the impact alone, and Fire Orb/Combustion.
For ACV, the Mahaon pulse gun if it's the killing blow. It's a monster but it wrecks your EN, so to get a shot off and on target is really satisfying.