Mostly between 10,000 and sea level. There was no set altitude because of the difference in performance of aircraft (like an early WWI fighter that was barely able to fly to begin with to high performance aircraft that were designed to combat high altitude zeppelins); as well as the different combat scenarios (how much time was available to climb to altitude, how much altitude was sacrificed for performance, weather, etc etc).
Most WWI fighters did not have boosted engines, which meant they had about 2/3 of their power at 10,000 feet. Most WWI aviators did not have oxygen, so would feel the effects of partial pressure hypoxia. Flying very low would give you better performance, but lower endurance due to the higher fuel consumption and you are vulnerable to obstacles and ground fire as well has have no altitude to trade for speed to pursue or escape.