I response to all this I will chime in:
First the Altar UFO:AfterX series are not related to XCOM in any way really other than the idea that humanity is attacked by aliens and we manage to fight back. But unlike XCOM where humans tend to win a lot and lose little, in Aftermath you start off with the majority of Earth decimated by the aliens. Believable. You only have a chance fighting back because most of what is left of the survivors is centered around military installations and military tech, and you actually go to Area 51 for the 'Dreamland Files' to help fight the aliens on top of reverse engineering their technology. This is where Altar was genius, while the aliens will kill you easily with their shields and weapons against your human tech but the game starts you off against mutants, results of the biological agent infecting Earth. This gives you starter enemies that you can take on and win against in the beginning, although you still have to play smart against most of them. Aftershock does the same, until the Cultists show up you are mostly mutant-bashing. In both games once you get used to dealing with the mutants you then go against aliens with similar or better equipment and really need tactical thinking to start pulling through missions. This difficulty scaling let's you get a good start without being as cheesy story-wise as XCOM in letting you kill hi-tech aliens easy right off the bat. UFO: Afterlight starts you off fighting weak aliens and ancient robotic Martian drones.
As for XCOM, we currently have the remake by Firaxis. It's simplified but still quite good really. I only like the first XCOM Enemy Unknown in the older XCOM's. TFTD and that 3rd one did not interest me much at all. TFTD was a good idea, alien sea bases in real life are much more likely since it's easy to hide there and we haven't explored much of the ocean at all. That and the Norwegian Navy has officially documented chasing USO's (unidentified seaborn objects) that were going way faster underwater than anything human then watching this USO go into the air as a UFO, with multiple ships and their crew witnessing it. But, big but, TFTD as mentioned by someone else had terrible artwork and in general the game was just a more difficult rehash of the original. Gauss guns = laser from first game but needed ammo etc. it was same stuff different names. The new XCOM is actually quite good in comparison and has an intuitive cover system. Not a bad game at all, I just wish it'd let you pick what class a solider will be instead of relying on random chance to get the sniper you want.
Xenonauts is pretty much done now, too. This is basically just a modern re-hash of the original XCOM. It's more like the first game than the Firaxis reboot by a large margin. Runs at modern resolution, decent graphics and not expensive. I'd recommend it, and the dev team is still actively patching the game.
UFO:AI is an open source project that has been going on for years and it's in a playable condition now. It's free to download and actually good fun now, but development has been slow since it is free and everyone working on it are volunteers donating their free time to the project. Considering it's a modified version of the old Quake2 engine, it looks really good. New higher-res soldier models are needed but it doesn't look as janky as it used to. This is worth a try for sure.
To sum it up, I'd suggest UFO: Aftermath, UFO: Aftershock, and UFO: Afterlight the most. Aftershock is my favorite by a long shot, it had global management and still had the gritty feel like Aftermath but has more stuff. Aftermath has random generated maps, the 2 others do not. Afterlight is the most modern and runs in widescreen but has the cartooney graphics, and is different as a whole in the geo-screen management but the actual in-game missions still play the same. All 3 are good. By the way UFO: Aftermath was one of my first PC games, I bought it like right when it came out new and it ran so slow I learned I had to get a graphics card upgrade to make it run smoothly. This game specifically led up to me eventually building my first full PC by the time I was 10, that and the gritty military sci-fi has left a major impression on me that I appreciate to this day. Good stuff.