Very interesting questions! To be perfectly honest, I feel kinda intimidated trying to answer it, because of how broad and difficult they are. But I'll do my best.
In my opinion, there are two top-tier tactical games about repelling an alien invasion (Jagged Alliance 2 is great too, but it's something else). These games are: the first X-Com (and by extension, OpenXCom) and UFO: Aftershock. These games are both incredibly entertaining, but also quite different, so I recommend everyone to play both of them.
Why not other titles you mentioned? Because I think they are imperfect clones of X-Com. They do have their strong points, but ultimately fall flat as games. Let me elaborate:
1) UFO: Extraterrestrials. This title was made in the darkest hour of tactical gaming, where the genre seemed effectively dead. That's the only reason this game reached any level of success, because frankly it was pretty shit-tier. It had some nice ideas, I actually liked the terrain for example, and the RTS-like air combat was engaging, but it was produced by a really small, inexperienced studio and it screams "made by amateurs!" at every opportunity. Its interface is clunky, the graphics are mostly bad, but the biggest problem is that the overall design was disappointing. It was just weak as a whole, from bad design decisions (aliens get worse with your progress, each tier of weapons was worse than the previous one except for damage etc.) to the snorefest finale.
Now, there is a huge mod for this game called UNIMOD which make it a much more enjoyable experience, with many fixes, additions, better balance, etc. But it didn't address the biggest problems in the game. It made it a good gaming experience, but not as good as X-Com.
2) Xenonauts. This game was also quite budgetary, but much more professional than Extraterrestrials and it looked very nice. But the core mechanic was actually the worst of all titles discussed here. All the good points were parroted from X-Com, and everything else was ultimately as nice as pulling teeth - the suppression mechanics, stupid armours, never-ending chases after teleporting enemies, unsatisfactory modding engine and so on. They clearly lacked a talented lead designer who would eliminate core problems with the game. I never forced myself to finish it. It has a really nice air combat model though which I enjoyed a lot.
Now, let's move to the X-Com proper. Why am I saying it is better than the other two? Because it was actually written by a great designer. It is perfectly balanced, has many ingenious mechanics (for example how reaction fire works - without any random element but very naturally), delivers exactly what the player wants, avoids tedium when possible, and so on. as a player, you just feel that you are in the right hands, and it's not nostalgia glasses - you are actually being catered to, without being belittled by the game. It is simply very polished, a high quality product. This is why I keep playing this game after so many years and so many other games - it's just the most fun.
Now, I understand your reservations about details like weapon selection. but the beautiful thing about OpenXCom is the modding community. I doubt many veterans here play exclusively vanilla X-Com; we've done it to death over the last two decades. But there are several huge mods which add and rebalance weapons, include new mechanics, polish the interface and so on. If that's what you're interested in, then OpenXCom is even more recommendable for you.
I'm not sure if this post was explanatory enough, but don't hesitate to ask more questions if necessary.
