On normal I used the assault rifle for everything except a rare few shotgun or sniper situations.

On hardcore and up I found myself using the full set. Sniper rifle to take down a couple of enemies at range if outside, assault rifle for the bulk of the killing work, pistol to fill the gap on the frequent occasions my assault rifle got targeted by that ability which made it unusable for a set period of time, and shotgun with knockdown ammo for dangerous foes like Krogan or the creepers or if my pistol also got knocked out of commission.