Which ingredient gives Chicken Pate its flavor?
The flavor doesn't come from one thing, but from the sum of its parts. In the recipe you posted you have many different ingredients, for instance sage, mace, shallots, and brandy. All these add complexity and depth, and combine to make the flavor. 

As for what is it that makes you personally like pate there's no possible way to answer that on this forum, you'd have to experiment. What I'd suggest is to make the recipe over a few times but leave one of the ingredients out each time and see which of the ingredients makes the most difference to you. If you do that please post the results, I'd be curious to know.