Planescape: Torment (1999) is an old cult classic and it's easy to know why. This is the best game that Black Isle was capable of and it takes place in the D&D universe, even if it doesn't look like it.
The first thing that happens in the game is that you wake up on a slab in the mortuary where the Dustmen work to prepare the dead to be buried or cremated. The game takes place on an Outer Plane which is a dimension outside of the normal D&D settings (which are called the Prime Material Plane) and there ideas become reality. Alignments are powerful and the reality can be changed with the power of will alone.
Unlike other D&D games you don't get to choose a race, class or name. Instead you play as an incarnation of someone else and you're tasked to figure out who that person was. You are allowed to choose your attributes however and unlike other D&D games you really need to put points in attributes that help you in conversations since the plane you find yourself in is filled by other, more powerful entities that you will have a hard time defeating in combat.
As you fight (or sneak) your way out of the mortuary you get a hang of how the game works. You start to understand a little of what the game really aspires to.
The greatest advice I can give you is to not look at any spoilers about this game at all since the game is literally filled with characters, objects or situations that serves to drive the plot forward. Even a hint of spoilers will ruin your experience of the game.
The unfamiliar setting, characters and the design of the game is superb. The only flaws are a non-existent party AI, lack of space (as the game seem filled to the brim with stuff to do) and the seemingly lack of other planes to visit despite the one you wake up in. Some characters also seem to streamlined or cliché but I cannot describe that without spoiling the game.
It deserves a (5/6), perhaps even a (6/6) if the game didn't have so many flaws. If you like role-playing games you MUST buy this game as it marks the ultimate form of the genre.