Vampire: The Masquerade - Bloodlines
A Source-engine based game that had the unfortunate happenstance of being both a bug-ridden mess, as well as being released on the same day as Half-Life 2. However, once people patched it up years later, it got picked up again and is now seen as a lost gem.
If you like rich lore, a good story, unique side-quests and a variety of play styles that dramatically change your experience... Play it now, but make sure you install the all-too-needed fan patches.