XCOM: Chimera Squad
If you want a cheap XCOM game that you can beat quickly, whilst still expecting a level of tactical planning, this is definitely a good buy.
XCom
XCom 2: War of The Chosen
I have beaten XCOM 2 multiple times so far, and this DLC adds a whole new set of scenarios. It enough for me to beat the campaign for a third time and still feel like I did something new and exciting.
XCOM: Enemy Within
This expansion add several things like cybernetic, psionic, and genetic alterations to you soldiers to give them that extra ege in battle.
