Yeah the first X-Com (just called UFO) was fucking fantastic, but when TFTD came out, it solved a few gameplay problems that you just couldn't go back to.. I forget what exactly, but I remember wanting to go back to UFO but not being able to force myself because of the annoying gameplay issues... The SURPRISE of walking around a corner and BANG there's an alien in front of you... In the dark, late at night, with the creepy music... That's what that game was all about.
http://pc.ign.com/articles/793/793534p1.html
That is the best/latest possible sequel (there have been maaaany attempted) and it just came out recently, few months back I think.. Look at the reader ratings at that site.. they gave it a 7.4, but the readers gave it a 9.6 over 1,120 votes.. insane. I might have to check it out!