I would still (after 40 some years and many sci-fi games) use Classic Traveller for this.
Of the ones you mention, I've read all of them except Alien, but only played Death in Space. Here are my, probably slightly idiosyncratic, takes:
The reason I've not bothered with Alien is that I'm not a fan of RPG's licensed from other media. They're always filled with lots of pages of fluff I either already know or don't care about. I love Free League's production values, but I am not a big fan of their rules. I doubt Alien is lightweight, especially compared to the others you mention.
Mothership seems very tightly designed, but there's quite a lot of setting deliberately implied in the rules (as opposed to laid out in fluff). That's fine; but its particular implied setting seems heavier on the horror side and lighter on the blue collar/'just scraping by' side to me. I prefer the latter, and like a bit more flexibility anyway. I also saw this
very negative 'played-it' review. I can't vouch for its accuracy, but it seems to raise some valid mechanical concerns.
I enjoyed Death in Space in a beer-and-pretzels way; but wouldn't play it again. Again, the rules have a lot of implied, but only lightly fleshed-out, setting. In particular it has a fair amount of idiosyncratic transhumanism and 'space magic' that wouldn't be easy to cut. Compared to Traveller's modular design, its rules are just not very adaptable. They also felt incomplete and kinda sketchy in play. In particular, the setting of the book's sample adventure (very human grimy-industrial) doesn't particularly match character creation and some other rules (which are very transhuman-weird). It just feels like a bunch of cool stuff thrown at the wall without much consideration of how it fits together. It's also not OSR in the sense of being a close clone of old D&D, so its not particularly cross-compatible with anything else (apart from in the sense that any game
could be adaptable to anything else). OSR-compatibility is useless for sci-fi anyway. The number of old Traveller modules is orders of magnitude higher than the amount of TSR-D&D scifi. Overall, this was a big miss for me, and I thought I would love it. The book is gorgeous and some of the random tables are cool, but I gave my copy away.
My main problem with Those Dark Places is the writing style, to the point I found it hard to give the game a fair chance. All the rules are presented in-game-universe, which is maybe cool in concept, but which I found extremely grating in execution. The chatty, verbose, tone just rubbed me up the wrong way. That said, the system looks simple (1d6 based) and elegant enough; and it nails the gritty vibe perhaps best of all the ones you mention. Beautiful art too. There's a sequel,
Pressure that expands on it and looks similarly pretty; but I haven't bothered with it due to my issues with the writing.
Hostile falls, just a little bit, into a problem that a lot of Cepheus games have. If you don't know Traveller, then Traveller itself (whether Mongoose or Classic) is a better written introduction to the system; but if you're already familiar with Traveller, then Hostile doesn't do a ton that you couldn't houserule (or that wasn't already done in in an early-80s magazine article, to be honest). That said, Hostile suffers this problem less than most because Paul Elliot is a pretty good writer. The real value of this game is in the huge (300+ page) setting book that is far more complete than an of the others discussed here (except maybe Alien). If that is something you would like, Hostile is probably the best option. The Rules are just lightly-modded Mongoose Traveller: perfectly servicable but I would prefer Classic for this (see below). This game a lot more complete than the other systems you mention: it has specific rules for different environments, different vehicles and so on. They system is modular though, as is the Traveller way, so the core is light and you just add what you want. If you go for Hostile, I reccomend also getting the free Mongoose Traveller starter PDFs just to help you get used to the system.
As I said, I would go with Classic Traveller. Its core is extremely light, but it has modules for everything, so you can add the systems that work for you. In particular, it's lighter than Mongoose Traveller or Hostile. It's also lower-powered (you won't have anywhere near as many skills), more brutal (you might not survive character creation and that sets expectations right for the rest of the rules), and gritty (unlike most of the games discussed here, you're not likely start with a beat-up, barely functioning spaceship: that's something you'll have to work towards). If you're not used to old games, as opposed to 'old-school' games, the rules might seem strange at first. There's no central skill mechanic, so read all the skill descriptions carefully and internalise the approach that Miller is pointing to in them. In essence, this is to roll 2d6 against a target number (usually 6, 8, or 10) and referee decides what mods are appropriate to that particular test. Maybe your skill points each give a bonus of one, maybe they each give a bonus of two, maybe a particularly high characteristic is similarly relevant. The ref decides (with player input).
The advantages of using classic Trav are simple. It has more support than all the other options
combined by many orders of magnitude. This is a game that literally did most of the stuff you would want before the designers of some of the other games were born (stats for Alien facehuggers are in Journal of the Traveller's Aid Society #4 from 1980, for example). The core is as light as anything, but you can expand it as much as you want, and it has robust rules for almost anything. It's
free in PDF or dirt cheap in print. All you need to start is the core and maybe
Citizens of the Imperium for some gritty extra careers.
Thanks for reading my thesis!
