While vanilla Xcom EW is enjoyable, but flawed, I think the Long War mod makes it a worthy successor to the original xcom. The mod more or less restores the depth of UFO defense while being very innovative in it's own way, with things like alien progression based on an alien research system, providing passive resistance to panic in nations by fulfilling requests, a weapon system with a great deal of unique weapons and tiers that fulfill unique tactical roles while still fitting into the overall narrative progression of xcom technology and a bunch of other features. Long War, in my opinion, captures the spirit of UFO Defense while still preserving the depth and replayability that vanilla Firaxis XCOM lacked and providing a unique experience of it's own.