For me, its a tie between Sacrifice of Angels and THe way of the Warrior. Both have their own merits. The way of the Warrior showed u what was possible from a space battle perspective and the fact that DS9 (the station) had "teeth." This is especially evident after DS9 (the station) was getting it's but kicked by three Cardassian cruisers in "EMISSARY."
Sacrifice of Angels was the first time that we saw a full fledge space battle among the federation, Klingons, Dominion, and Cardassian. They set the standard for what was to come.