What games can you recommend that didn't get the appreciation that they deserved?
I've been recently been thinking about Arkane Studio's Prey which is a immersive sim, with a pretty good rogue like dlc, that probably has one of the strongest hooks of any game I've played. If you liked Halflife, System Shock, or Deus Ex it's definitely worth a play.
Are there any titles that might not have been commercially successful that you feel everyone should give a shot?
Far: Lone Sails is a beautiful art piece with unusual gameplay, and the sequel is great too.
Bedlam is kind of a love-letter to 90s and 00s FPS games. The gameplay isn't amazing, but if you spent a lot of time in games like Quake, Unreal Tournament or Halo CE back in the early days of online multiplayer, this game is for you.