I absolutely adore these games so therefore have to throw in two pence, even though it is likely to contribute naught in the way of originality.
I always play apocalypse on real-time mode (which seems to be scorned upon by turn-based purists) and always select the easiest option on whatever game it is.
Bought the set off of steam, and was absolutely stunned to see how well UFO: Enemy Unknown has aged, and probably remains the best of the lot.
Back on the Amiga version me and my Brother discovered the cheat where you could produce one thing at a time for no cost. We became self-financing solely on the production and sale of laser tanks.
Earth still pulled the plug on us because we were absolutely terrible at the game.
I was pleased to see that I am still pretty bad at it, and haven't played it for a few months as the last mission I played was a downed Snakeman terror ship. The Chrysalids shit me up proper, man. Half my force became those chitin encapsulated bastards and I went through virtually all my Heavy Plasma rounds just fending the swarms off.
Eeek. The horror of the chrysalids.
Despite how much I enjoy it, I feel that X-Com 1 hasn't aged well, mostly because of needing to re-equip everyone every battle. If you like Apocalypse on Real Time mode, you might be interested in Cenega's Ufo trilogy (Ufo Aftermath, Ufo Aftershock, and Ufo Afterlight). They are spiritual successors to X-Com, all with real-time combat. I think that they have playable demos.
I didn't particularly care for Aftermath. The strategy part is downplayed, but not every alien needs to die in every battle, and there are other human forces (Invisible, sometimes completes missions). There is unlimited ammo for basic weapons (Pump-action shotguns, Colt .17s, and frag grenades, I think). Aftermath is the only one in the series with ufo interceptions.
Aftershock is probably my favorite from their trilogy. (STARFORCE WARNING!) The player forces are in a big ufo-base thingy, and builds up many small bases on the ground, while amassing various resources. Making the enemies go boom is, of course, mandatory.
Afterlight focuses on the colonization of Mars. (Securerom warning.) Aliens can be recruited into the player's ranks, adding fun. Sadly, the alien recruits are squishy and die easily. One of the largest complaints about this game is the lack of ability to recruit new soldiers, making iron man games very difficult to pull off.
Unfortunately, neither Aftershock or Afterlight worked quite right on my computer right after installation because of DRM not liking my CD drive, forcing me to download unofficial fixes. They worked great after that.
While I'm advertising X-Com-esque for no apparent reason, Ufo Extraterrestrials was made by a different company. Dead soldiers are revived after a stay in the hospital, and for some reason I can't quite be bothered to beat it, despite it having flamethrowers.
Ufo: Alien Invasion is open-source freeware currently in development and playable. Interceptors are shot down depressingly easily, as I recall. It does have mutliplayer tactical combats though. I'll stop now.