The best game ever made is undoubtly X-COM terror from the deep. Some argue the first one is better but it's not true. While it laid the mechanics, the presentation, detail, difficulty and TERROR of the second game is unmatched. I have played Undying and was scared, I have played all the Dead Space series and they were tense... But no fear in the world beats the terror the TENTACULAT causes, nothing in the world is more horrible then your Commander dying to a bio-drone self destruction, no other game where you hold you breath during enemy turn, staring the "HIDDEN MOVEMENT" screen helplessly, praying your second-in-command survives this turn, because he/she took a %95 shot and missed and left facing a Tasoth. And the Moleculer Control of the aliens, which attack your soldiers brain functions and turn them against you!
I must mention Tentaculat... This guy has a ton of time units, incredibly high reflexes - so you can't easily reaction fire them- can't insta-kill them without heaviest of sonic weapons which are high in the research tree, and most importantly... they float near your soldies, they attack them in melee. and turn them into zombies. The dead body is controlled by injected parasite, attacking his fellow teammates. Not only the remaining soldiers need to shoot their beloved friend, when they do, another tentaculat is spawned from the body... And the nightmare cycles begins anew.
Before you know, you are trying to solve a giant puzzle with lots of mechanics... trying to stay in cover, do the mission objective, capture live aliens for research, fight off tentaculats, incredibly though lobsterman, mind-controlling tasoth, self-destructing bio-dromes... all the while managing your bases, research, production, economy, fighter subs... fighting against the aliens seemingly unstoppable spread over the globe, intercepting their missions, destroying their production and control facilities which they seem to build two for each previous base they have every month... and you have to satisfy your global funding countries. And you WANT to walk into that chamber of torture willingly! The game was not hard for the sake of being hard. It was hard to forge you into perfection. And you YEARNED for that perfection.
Then one day, you grow balls and decide to all of these in superhuman difficulty...
There were so many hidden, tiny details buried in TFTD... Like the tiles your men step on make different sounds based on their material, like the many small factors used to calculate panic... did you know being on fire increased panic probability? And the way skills improve, and the way you get attached to your men... And the way you design your base, and later aliens raid your base, and you play the game inside the base AS YOU DESIGNED IT. And on higher difficulty levels you needed to include base design in a tactical viewpoint as well... It was the perfect balance of art, style, variability, difficulty, complexity, presentation and satisfaction. If I had the power, I would have forced everyone in the world to play it.
There are other good games but next to TFTD they are just "mentionable", The new X-COM is quite good and a realy good challenge, but the TERROR part was removed.