I used to play the base game's multiplayer. A LOT. But that's mostly fizzled out due to netcode issues and imbalances.

However the single player still remains.

XCOM: EU/EW are both pretty good single player experiences, but pale to XCOM 2, as you might expect from a sequel. Plus both games have their fair share of issues that can be exploited either intentionally or accidentally, so mods are usually what make XCOM XCOM nowadays.

Long War is what most people play when they consider playing the first XCOM nowadays, but that also shares its own problems that have never been fixed due to the developers moving on to XCOM 2 and their own projects outside of XCOM.

Long War Rebalance on the other hand? One of the best strategy game experiences I've ever played, and undoubtably the best version of Xcom, including Long War 1/2, and Xcom 1/2 in their entirety.

The game is no longer about tip-toeing trying to not set off pods; LWR encourages big firefights by chain activating any alien that the aliens see when they activate and after each turn (which makes retreating aliens much more of a threat...), and also gives your troops much more resilience through health and DR so they can take a few hits in most cases. Plus shivs are much more useful early on because of their incredible tanking ability, so you always have a tool you can use to extend fights while minimizing danger to your own flesh and blood troops.

You are encouraged to play faster because the fatigue mechanic gets more severe the longer you're out on a mission, so again, no more tip-toeing from place to place for fear of activating. XCOM gets their own scatter too; any time you activate a pod, any unit with no action points gets a free movement action to reposition, again making it safer to play faster while encouraging the firefights.

The air game has been completely overhauled with the integration of another interceptor overhaul mod and some rebalances, making it much more interesting due to sending out multiple interceptors at the same time in different formations.

With LWR, an issue in both Long War and the original game is completely solved: the reverse difficulty curve. Now the game appropriately starts fairly easy (but still challenging) but actually gets more difficult as you go along, rather than the new tools you get making the game easier as it goes on.

And beyond all of this stuff, the developer of LWR likes to throw out updates every week or two, often with a complete overhaul of a previous mechanic and some significant balance changes. Long War Rebalance is more of its own mod now than just simply a rebalance like the name implies, and it's most certainly the biggest mod that Xcom has to offer.

If you don't own Xcom? Do yourself a favor and beat the base game once or twice then try out LWR. If you do? Go try out LWR immediately, you won't regret it one bit.