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?
I was mildly a Borderlands fan, but then I played Tales from the Borderlands and fell in love. It's such a great game with amazing writing and music that I'm always surprised to hear that most people, including fans of the main Borderlands games, have never heard of it.
That's one I did play, the only other Borderlands I finished was the second but I think it did a good job of keeping what made Borderlands Borderlands while going to a completely different genre.