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?
A lot of the games people have mentioned here are either obscure games I've never heard of or newer titles in niche communities. But Gun Point is an obscure game I have actually played, I think they could have picked a better name for it though for a game where most combat isn't firearms based it's slightly misleading and probably deterred some people.