xcom1 is a classic there is no reason any serious gamer would not have that on a desert island usb drive with dosbox. It's that good. I think this is the only one openxcom works with.
xcom2 TFTD is xcom 1 rehashed. Most everything is the same just different sprites. But it's brutally hard. Rumor is that the old xcom1 difficulty bug where it always got reset to beginner was not discovered by the time it was made and gamers complained that xcom1 was too easy. So xcom2 on beginner is equivalent to xcom1 on superhuman.
xcom3 Apoc is in my opinion an improvement on xcom1 in most aspects. Collapsing structures in the battlescape, more unit and weapon variety and even the ability to actively attack opposing corporations(which are like countries in xcom1). Apoc has some downsides though, you are protecting one city, not the entire globe, alien ufos attack in waves so you have very long periods of time where nothing is happening and you KNOW nothing is happening, between periods of "Holy crap how am I supposed to deal with all this". I personally feel it is better than xcom1 overall, but feels distinct enough to still want to play the first one about the same. If open xcom worked with this I would not be seen outside my house for months.
and enforcer and interceptor I haven't played. But they both sucked by popular opinion. In enforcer yer controlling one guy in powered armor the whole game, and interceptor is a space combat game.