Christmas sales have ended, but there are still a few bargains. The XCOM: Ultimate Collection contains XCom 1 'Ennemi Inconnu' + XCom 2 + Chimera Squad + all of the DLCs. It is a proper bundle so you will be discounted for games and DLCs you alreday own. Since some of the DLCs aren't discounted at the moment, the bundle is definitely the cheapest way to obtain them.
There are some mods available for the games, if you like to depart from vanilla gameplay and visuals. My brother insists that "real men" play X-Com: UFO Defense, the original game from 1993 (available on GoG). If you cannot stand 8 bit era graphics, Xenonauts is probably a good 2d X-Com alternative.
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