For me it's Sword of the Stars 2. If you know anything about the game at all, you know why this is considered a 'bad' game by many.
The original Sword of the Stars (released in 2006) was a space 4X game, that mixed turn-based strategic play, with RTS fleet battles (confined to a 2D plain). I've never played it, but I gather it was pretty well received, with a metacritic score of 68.
The sequel tried to make everything bigger and better than it's predecessor. However, to say the game was let down by a problematic launch would be an understatement. I didn't pick the game up until a while after launch, but as I understand it, the game was released in a pretty much broken, alpha state. Thankfully, the developers were quick to provide patches after launch, and continued to improve the game, but it took them until almost a full year after the game's release for them to announce the 'all-clear', that the game was in an acceptable, release-worthy state. Around that time, they also released an expansion, free for early adopters of the game, that added a seventh playable race to the game.
These days, the game is in a mostly playable state, however there are still a few bugs and crashes here and there, and even on hard difficulty, with a starting advantage over the player, the opponent AI is still kind of useless. Unfortunately, some time after the 'all clear' the publishers, Paradox Interactive, pulled funding to the developers, so the flow of patches has dried up (game was last updated November, 2013).
After all that, Why do I think it's a good game? There's obviously something in there that I like, as it's currently my most played game on steam at 228 hours total playtime, beating my second most played game, Skyrim, with only 156 hours total.
It's not a feature unique to the game, but I'm a sucker for games that let you customise your units. And SOTS2 scratches that itch very nicely, you chose from three sections to build your ships (command, mission & engine), and then customise it's armament, down to what weapon is mounted in each individual turret.
Again, randomized tech trees aren't unique to this game, but the way the game does it is quite interesting. While there are certain core technologies available to every race, most of the more interesting and powerful techs only have a % chance to be available to you in a game, with the chance determined by your race. However, you don't immediately know whether or not a tech is available to you or not, you have to unlock it's prerequisites on the tech tree, and then perform a 'feasibility study' on it, to see if you can research the tech or not. Which helps keeps games interesting, by forcing you to change up your strategy, for example: Maybe in one game you use Phasers very effectively, then in the next game, you go to research phasers again, and you fail the feasibility study. What do you do now? Maybe try Heavy Combat Lasers? or perhaps torpedoes?
While it's not really part of the game itself, and most of this info is found in the lore forum on the developer's website, or in the in-game 'sotspedia'. I really like the lore of the universe that they created, they try to do something different with all the races, rather than just following science fiction tropes. Take, for example the insectoid Hiver race, rather than just being another alien hive-mind, they are actually ruled by an absolute monarchy, with a complex political and caste system. If that's not your thing, maybe you'd prefer the tribal meritocracy of the Morrigi Confederation (who also happen to be a race of telepathic space-dragons). Or maybe the psionic cetacean species, the Liir. Yes, this game has whales with psychic powers, in space. Don't laugh, they're bad-ass. I always end up slightly role-playing the leader of my chosen race whenever I play a game of SOTS2.
Yeah, so, the game's by no means perfect (in fact, far from it), but it's definitely a diamond in the rough. Go check it out if space 4X is your thing.
