My favourite XCOM is XCOM: Enemy Unknown*, a pure and direct, no mess/no fuss modern-day remix of metagame-augmented turn-based tactics. My second-favourite XCOM is the polar opposite, XCOM 2 DLC War Of The Chosen, an absurd explosion of superheroics that throws internal logic to the wind.
Naturally I poured hundreds of hours into XCOM 2 itself, but something about it never quite clicked in the same way XCOM had. Its rebels vs future utopia scenario was inconsistent, and it couldn't quite seem to make its mind up whether to be a gritty game of stealthy guerilla combat or cartoonish maxi-sci-fi. Expansion pack War Of The Chosen throws all such caution to the wind, and embraces ridiculousness with wild aplomb, while also being satisfying complex and variable battles.
Slightly blowing my own murder-trumpet here, but when I think of WOTC I think of this insane first turn I pulled off with a high-level squad. Dozens of deaths at the hands of a single soldier, and one round that lasted the length of a standard mission. Nothing in this is breaking the game's rules in any way: it's simply the ultimate outcome of piling all the wild skills on top of each other, and a deep understanding (built across dozens of hours of play) of how they combine.
Sure, WOTC's own tone is all over the place, but I love it for being such a toybox of possibility, where agonising about what to do isn't about picking one of three or four possible actions, but dozens. I don't know if turn-based strategy has ever gone as big as this before (and I'm not sure I want it to make a habit) but War Of The Chosen is a wondrous apex for now.
* Though, clearly, my favourite X-COM is UFO: Enemy Unknown.