Here is my summery of these games, as someone who has touched all of them at some point (and either bought on here, Steam, or physical copy). Just writing this because I love Space games :).
No Mans Sky: Personally I've found it an excellent experience even prior to patches in a same way going outside for a walk feeling a fresh breeze is excellent. Even with all those added gameplay elements due to constant patching it certainly is not a game for everyone. If you like walking a lot or going to different places and games that are slower paced than Minecraft you will love this game. If you like action/story based games or fast paced games that require quick reflexes like Terraria then stay away. No Man Sky is the perfect game to play on a rainy day, in my opinion.
Stellaris: Having played around 80 hours, I think that Stellaris is probably the greatest 4X game of all time (above and beyond MOO2 IMHO). But like most great games, its the mods that elevate it. Even if you just like RTS games but don't like 4X games I think its worth a try on a discount.
Freespace 2: Greatest space game of all time with some of the greatest (record beating) mods of all time. Probably worth several GOG blogposts IMHO. As the game engine got open sourced decades ago, there are many great things added to the game, that should appeal to gamers both old and new. For starters, buy (or get gifted) the game, install it inside your directory of choice and then get
Knossos and just follow the instructions for the potentially best experience that you can have with the game. Have fun! (I wrote this last year but I still intend to make a huge FS2 giveaway happen at some point regardless of the free GOG-goodness they're doing right now :)!)
Wing Commander series: Freespace 2 might be one of those flawless games, but it probably wouldn't exists without the Wing Commander series, which paved the way for many of the other space games to come and certainly made the whole subgenre atmospheric games more mainstream friendly, especially with the use of Full Motion Video later.
Dead Space: The first Dead Space was so scary for me that I couldn't finish it and thats a hell of a accomplishment and I am not usually scared easily by a game. The other Dead Space games aren't really scary in my opinion.
X-Series: Comparing the X Series with Bucket Wheel Excavators seems apt. They're both huge, very bulky with lots of things that makes them function, are very slow but extremly highly sophisticated and respected by everyone who had experience with them.
My personal X ranking, top is highest ranking:
X3: Albion
X4 - Foundations 2.0 (I wait until DLC release and the next large patch)
X3: Terran Conflict
X2
X-Tension
X1
X3: Reunion
(I forgot about X-Rebirth, but I haven't played as much of it to place it anywhere, after years of updates it certainly ain't terrible anymore. You can think of it as a more Sophisticated X1 now.)
Homeworld series: Last but not least, its good old HW. I love those games and I own all the games on physical copies. I still play some of the mods every once in a while and they're still an excellent time. What not many people know, it also has a lively chinese community that I do enjoy reading up on, so with that in mind it has for sure a very diverse community. I can conclude that even just HW2 can universally captivate the minds of many people and that is probably the biggest praise for any game and the devs that I can voice and a hell of a huge achievement. The Remastered version for both the HW games and Cataclysm (called HW: Emergence now) are a must have for anyone using GOG.