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?
Aww man, I loved Heat Signature and Gunpoint both. They definitely deserve some more love.
Both games are by Tom Francis. His new game Tactical Breach Wizards is 'coming soon' as well
An another one to add in the defenestration trilogy :D
Maybe I should replay gunpoint sometime