Even with some of the publicity on mainstream sites and cultural relevance within our medium, I still find System Shock 2 to be severely overlooked even now. Not so much the original which has baaaaad but the sequel is everything I want in games. It has depth, it is a mechanically amazing game and while not pants-wetting, it can be surprisingly harrowing- giant invisible (they fade into view when they get close to you) spiders the size of a truck tire that inflict damaging poison on you being the supreme highlight.
Honestly I feel that to this day, the big issue is that even though the game is 10 bucks on average, usually on sale for lower, more people don't play it because of the platform it's on, period. Which is depressing since if you have a PC made after 2001, you can play the game. I have played it on junk desktops and it ran just fine. The other issue is modding- a lot of people still have an ill opinion of modding and as a computer science grad (who mostly plays on consoles to be fair- by the end of the day I'm sick of computers), I've always loved modding but most people can't hang with it even though by this point, SS2 modding is supremely streamlined.
But that's all my opinion. My other dark horse game is Silent Hill 3. I won't go into depth on it, but when most people think of Silent Hill, they think of the second game and the movies. But the original game (I love it still but it's aged poorly) and the third game are my favorite two of the whole property. Not being derogatory in the slightest- when feminists are looking for great games to play, Silent Hill 3 with it's unsexualized 16 year old female lead, it's unsexualized female villain and no cheesecake or romantic trash in it is the game for them. I always see the Japan-only cover of the third game's soundtrack brought up against the game and again, that was Japan-only and nothing in the game resembles it (because it would be illegal- the female lead is 16). But the game is my favorite horror games and it doesn't get the love it deserves- I know SH2 tells a more personal and dark story, but I've had family members die of cancer already, I don't need a horror game to tell me how much that shit sucks!