sadly the latest 'sequel' looks utterly terrible. lets take an excellent tactical turn based squad shooter, with research and development, and rpg bits, and remake it into what looks like a generic FPS (and it doesnt look like its very good even at that). yay.
TFTD was fun, but very much like EU - the research paths were near identical, iirc.
Apoc was pretty good, and i liked how they progressed the storyline, and kept similar gameplay - whilst updating it for a more modern audience.
interceptor... i quite liked interceptor. theres not many flight sims i like, but the combination of a fun storyline, and the R&D stuff, made it pretty much unique as far as i'm aware.
enforcer... i played once, and it was awful.
now, as for the 'spiritual successors'....
ufo: afterlight/aftermath/aftershock.... not bad games tbh, but didnt have the charm of the originals. also fairly buggy and unresponsive iirc.
a few people have mentioned ones above, which i wont comment on as i've not played them.
i have been playing UFO:Extraterrestrials lately (and a sequel is out soonish), which captures a lot of the the essence of the originals, imo. the feeling of being utterly outclassed is certainly there, along with the research and your squad improving (if they last long enough!). controls are a bit iffy, and theres a few bugs, but nothing gamebreaking.